After a chamber where you disable turrets and use momentum to put a cube on a button, you reach Test Chamber 19.
Put a portal on the angled piece of wall that doesn't have a pellet bouncing on it, then put a portal of the other color on the angled piece of wall that does have a pellet bouncing on it. This will start the platforms moving.
Ride the platform, crouching to pass under the metallic cylinder. Jump onto the grate, and continue riding the platform until you reach a small side room with a button in it. Go into the side room, but don't push the button yet. The button opens the nearby door, but the door doesn't stay open long enough for you to ride the platform past it. Create a portal on the wall near the button, then press the button and look through the door that opened. Make a portal of the other color onto the wall behind that door. Watch through the portal to see when the platform goes past, and go through the portal onto the platform.
There is a high-energy pellet up ahead. You can't crouch below it or jump over it, so make it go through a portal, and make a portal of the other color near the ceiling, and once the pellet goes through, make a portal of the other color next to the one near the ceiling. This should keep the pellet out of your way.
After you get past the pellet, look for a wall in the path of the platform. Make a portal on the wall just beyond the wall that is in your way, then make a portal of the other color on the wall that is in your way. When the platform reaches the wall, go through and you should be able to get back onto the platform as it goes past the wall in the way.
As you ride the platform, you will see a cake symbol on the wall. But as you turn the corner, you see something else, and the computer voice has a grim message for you. Fortunately, you have the portal gun. Wait until you can see the metallic ledge above the pit, and put a portal on the wall above it. Then jump toward a side wall and make a portal of the other color that you will go through as you fall.
You can see a ledge far above the pit of fire. Make a portal on the wall across from that ledge, as close to the ceiling as you can make it. Make a portal of the other color on the floor beneath you, then as you fall, make a portal of the same color beneath you, and you should have enough momentum to reach the ledge.
You find a side hallway. As you try to go up the stairs, they collapse beneath you, so use portals to reach the upper walkway. Go to the metal door and press the interaction button to open it. Go up the stairs and you find a locked mesh door. Make a portal on the other side of the locked door and go through a portal nearby. Open the metal door here and go through.
There are two large fans here. Make a portal on a wall behind one of the fans (it doesn't matter which one), then make a portal of the other color on a nearby wall and go through. If you are having trouble making a portal behind a fan, get as close to the fan as possible first.
There is a slope leading down to a poison pit. Across the poison pit, there is a ledge. Make a portal on the wall behind that ledge, then make a portal nearby and go through.
There is a walkway over the middle of the poison pit, and part of the walkway is missing a railing. Make a portal high up on the wall facing the part of the walkway that is missing the railing, then make a portal of the other color on the floor and fall in, then fall into the floor portal again to get enough momentum to reach the walkway.
One end of the walkway leads to an office area that you can optionally explore. The other end of the walkway leads to an area where you see an exit sign written in red on the wall. Make a portal on the wall where the red arrow is pointing, as high up as you can, then make a portal of the other color on the floor and fall through it repeatedly to reach the upper ledge. The computer voice will briefly speak to you.
Go up the stairs and you will reach a dead end with some flowing air coming through a broken tube. Jump into this tube and you will automatically slide through. You can't change your direction. When you stop, jump down onto the hatch and it will break open, and you will be back in Test Chamber 01.
Stand on the big red button and make a portal on the wall beyond the door that opened, then make a portal of the other color in this room that you can go through to get past the door.
Go toward the elevator and you will find that there is no elevator there. Jump down to the bottom of the elevator shaft.
Make a portal on the underside of the low ledge behind the fence, then make a portal of the other color on the spot where you landed when you jumped down the elevator shaft. You might have to crouch to get down from where you land after you go through the portal.
There are two pistons going up and down. Stand on the taller of the two pistons and you will be raised up to where you can see the top of the platform that you put a portal on to reach this area. Make a portal of the other color on the top of this platform (i.e. not the same color as the portal on the underside of the platform). Then go back down to the underside of the platform and crouch while jumping to go into the narrow space under it. From there, jump up through the portal so you come out of the top of the platform.
From here, go onto the top of the metallic cylinder that is going sideways, and go to the edge of it. Wait for it to extend, then go to the ledge with the red exit sign.
Go toward where the red arrow is pointing, and you find a dead end with a metal grate. Make a portal on the wall that you can see beyond the grate, then go back to the top of the platform next to the moving cylinder and make a portal of the other color on top. Fall through to reach the area beyond the grate.
Look up and you will see a ledge up above. Make a portal on the wall across from that ledge, as high up as you can put it. Then make a portal of the other color and jump (don't walk) through it to reach that ledge.
Keep going, and there will be a brief "Loading" message. Beyond that, you find an area with five pistons. Some of them go all the way up to the ceiling, so don't stand on them for too long or you will be crushed. The middle one does not go all the way up to the ceiling, so stand on it. When it raises you up to where you can see a ledge near the ceiling (there is a broken ladder leading up to it) make a portal on the wall beyond that ledge, then go back to ground level and make a portal of the other color that you can walk through.
From there, you will reach an area with two pistons. Look up and you will see a horizontal piston up above. Ignore the ledge above that piston, because if you come out of a portal on that ledge, you end up in the path of the horizontal pistons and could easily be crushed. Instead, look up above the ledge that the two vertical pistons are hitting. You should see a wall up above that you can put a portal on. Make a portal on that wall, then make a portal of the other color on either of the two dark marks on the ledge that a vertical piston is hitting. Stand on the piston and look up at the portal to go through.
Look for the red exit sign on the wall of this ledge, and look where the arrow is pointing. There is another red arrow pointing to a ledge. Make a portal on the wall across from that ledge, then make a portal of the other color on the floor and fall into it, then make another floor portal to fall into if necessary, to reach that ledge.
In this area, you see several high-speed pistons. Make a portal on the wall beyond those pistons (it may take several tries), then make a portal of the other color that you can walk through.
To the side, there are a couple of vertical pistons. When they are both out of the way, you can see the wall beyond. Make a portal on that wall, then make a portal of the other color nearby if necessary. Go through.
A sign on the wall in this area says "over here" with an arrow pointing up. When you look up, you will see several horizontal pistons. Make a portal above one of these pistons, then make a portal of the other color somewhere nearby and go through. You should be standing on top of the piston, and you won't be pushed off even when the piston retracts. Then make a portal of the other color (not the same color as the one that you can walk through) on the wall above a higher piston. From there, the only way to go farther up is to put a portal on the wall between the the two highest pistons, and go through that portal to land on the piston that goes below it. From there, you can see a red exit sign on the wall beyond the fence. Make a portal on the floor of that area, and go through a portal of the other color to get there.
Another Cube Tube
Go to where the red arrow is pointing and you will find a cube tube. Drop down onto the tube and walk along it. You reach an area with a poison pit below you, and a cube tube just above the poison. Land on the cube tube and keep walking. When you reach an area with a piston in front of you, make a portal on the wall above where the piston is hitting, and then go back to the poison pit. Make a portal of the other color on the wall above the poison pit, then wait until you see the piston extend toward the portal, and jump through. You should land on the piston, which goes to a ledge to a new area.
You will reach a door that doesn't open and hole in the floor. Drop down into the hole. There will be another brief "Loading" message.
You can optionally crawl into the vent and look around, but it is a dead end. Go down the hallway and you find an area with several pistons, but don't walk on them yet. You may wish to read the following first. When you walk on the pistons, they will move away, dropping you into a pit where three doors will open on a time delay, one after the other, each revealing a turret. There are several ways to deal with this. One way is to make a portal on the wall behind the first turret as soon as you can, then make a portal of the other color under that turret to make it fall over, and hide in the corner until it stops shooting. With both portals in the same room as the turret, you can keep the gunfire contained while you hide in a corner. Then when the next door opens, you can make a portal of either color underneath the second turret and hide in the same corner to avoid its gunfire. As soon as it stops shooting, make a portal of the other color in the second turret's room so that both colors of portal are in that room, and when the third door opens, you can safely make a portal of either color under the third turret and hide in a corner until it stops shooting.
After all three turrets have been disabled, look in the third turret's room. It has a claw hanging from the ceiling. Look up above that claw and you will see a red arrow on the wall. Look where it is pointing to see a wall that you can put a portal on. Put a portal there, then a portal of the other color that you can walk through, to reach that area.
Don't be afraid of the blinking turret here. It won't shoot you, even when it sees you. Beyond the fence, there is a cube tube. Make a portal on the ceiling above the cube tube, and go through a portal of the other color to land on the cube tube. From there, you can jump across to the metal ledge.
Look for the red exit sign on the wall and crawl into the hole there. You will see a turret down below. Disable it and fall down into the hole.
Move backward away from the hatch that you just came through, until you can see the top of the hatch. Make a portal on top of the hatch, then make a portal of the other color on the floor and fall into it, then when you are falling toward the floor, make another portal of that color to fall through, and you should reach the ledge.
Continue on and you find yourself in some back offices of Aperture Science. There is a door with a particle field in front of it. Go through the particle field and open the door. Press the red button, and a mechanical eye with a blue laser sight will awaken. Don't panic! It will slowly rotate until it has you in its sight. When its eye turns red, move out of the way. It will shoot a rocket through the glass, shattering it.
Go through the glass and circle around the rocket turret, staying out of its sight, until it's facing the glass on the other wall. Make the turret shoot through this pane of glass, then go through into that room. There is a door here, so go through it. But before you leave this room behind, look at where the rocket turret's laser sight is pointing, and put a portal there. Be sure to remember which color portal it is. You will need to leave it there for a while.
Now go through the door and make your way down the hall to another door. Open it and go through, then make a portal of the other color that faces the glass wall in here. Make the rocket turret see you through the portal such that it shoots through and breaks the glass that bars your way. You will still need to make use of the rocket turret, so don't place any new portals yet.
Go through where the glass was and you will see a cube tube, as well as an "over here" sign on the wall. If you go to where the arrow is pointing, you will see a vent that you can't reach by jumping. You can use the rocket turret to break open the cube tube and get a cube to stand on. Just make a portal on the floor beneath the cube tube, and carefully look into it without falling in, until the turret locks onto you. Move out of the way and it will break open the cube tube. Take the cube and put it beneath the vent, then jump onto the cube and press the crouch button to crawl into the vent.
At the end of the vent, you find a spinning fan. Make a portal on the wall (not the floor) below the fan, then crawl back out of the vent and make a portal of the other color that you can walk through.
Beware of the poison pit up ahead. Make a portal on the wall above the angled ledge that you can see up ahead, then make a portal of the other color to go there. Go to the far end of the ledge and there will be a "loading" message.
At the far end of the ledge, you can see a ledge across the poison pit. Make a portal on the wall beyond that ledge, then a portal of the other color that you can walk through to reach it. Go to the far wall of this area and you will see an arrow drawn on the wall. Look up into the small grate and you will see some walls and ceilings. Make a portal on one of those walls or ceilings, then make a portal nearby to walk through and reach that area.
There is sludge on the floor in this area that makes you slide when you try to move around. There are ledges up above, but there are turret doors up there that will start to open as soon as you go up. Quickly look around to see which turret door is about to open, and make try to make a portal behind a turret, of the color that you did not just walk out of. That way, you can walk into the portal that you just came out of, to end up behind a turret, allowing you to pick it up and drop it off of the edge to disable it.
Continue disabling turrets this way. You can fall down into the sludge pit to avoid the gunfire while you place portals to disable more turrets.
After all of the turrets have been disabled, use portals to reach the highest turret room. From this room, you can see an angled wall across from you. Make a portal on that angled wall, then fall down below and make a portal of the other color to fall into. As you fly upward, make a portal on the higher angled wall, then make a portal of the other color on the floor to fall into. You should reach an upper ledge at an area with metal walls and floors.
Carefully go forward and watch out for the turrets that are placed in front of you. Quickly make a portal on the floor behind those turrets, then run off to the side to avoid them. Make a portal of the other color on the nearby floor to fall into, so you end up behind the turrets. Disable them.
This is a dead end, but if you go back to the edge that overlooks the big room, you can look upward to see walkways far above. One of the walkways connects to the wall above the ledge that you are standing on. Put a portal on the floor next to where the walkway is up above, so that if you fly up from that portal, you will be next to that walkway and can move onto it. After that, fall down below and make a portal of the other color beneath you, then move onto the walkway as you go up.
Go to the far end of the walkway and open the door to reach some back offices. You will find a door at the top of a short flight of stairs. Go through and walk down the hallway, which has a particle field at the end. Continue on to find what awaits you beyond Test Chamber 19.