Shoot a portal on the ceiling over the floor across the pit, then use a wall portal to drop over to that side. Once you land, there is a tall white wall that you can see. Look at the ceiling above it, and shoot a portal at the farthest part of the ceiling. Then make a portal of the other color on the face of the tall white wall, below the level that you're standing on, so you can fall through it. You should land on a piece of floor with a two-dot tile on it.
Next, shoot a portal onto the ceiling above the ledge with the three-dot tile on it. Finally, when you can see the four-dot tile, make a portal on the piece of wall that sticks out, then make a portal below your feet to fall onto that higher ledge with the four dots.
Now make a portal on the lower piece of floor that you can see from the four-dot ledge that you're standing on. Fall into that portal and you should have enough momentum to make it to the ledge with the red button.
Don't bother with the red button just yet. Instead, go through the door and into the room with the small red button. Pressing that button will make the wall panel push out, giving access to the big room. Be careful, though, because there are turrets here, and at least one of them can see you if you emerge from the right. While the panel is pushed out, make a pellet shooter shoot a pellet at the various turrets until they are all disabled (there are four). If you want, you can exit from the left of the panel. If you position yourself correctly, none of the turrets will be able to see you if you stand to the left of the pushed-out wall.
Now that the turrets are gone, you have to put a portal on the piece of wall that sticks out at the far side of the room that faces the small room and small red button to the left of the ledge you're standing on. Make a pellet come out of that portal, then quickly put a portal on the piece of wall sticking out that is close to the small red button, and put a portal on a wall close to you. You need to press that small red button as soon as possible. You can probably reach the red button through the portal, without having to jump through. But if you want to try that, practice first to make sure. It's not that hard to just jump through and press it while you fall, though.
Once the containment thing has the pellet in it, a platform from your ledge to the other end of the room will be activated. Ride the platform to the far end, and fall onto the lower ledge. Pressing the red button here will temporarily open the wall panel that you entered this room through, so before pressing it, make sure that there's a portal over there, as well as a portal on a section of wall in this area. Then press the button and hop up through the portal. Go through the wall panel (crouching if necesssary), then look down from the ledge where the big red button is and make a portal on the far ledge down below.
Now go back to the small red button, press it, make a portal of the other color on the piece of wall that just opened (it's angled up), and quickly go back through the wall panel to the big red button. Drop down through the floor portal before the wall panel closes and you should have enough momentum to fly over the pit into the upper ledge in the room that had the four turrets.
There is a cube here. Take it and jump onto the moving platform when it gets close to the ledge. Drop it off on the ledge with the glass wall, and make a portal next to the wall panel that opens up. Ride the platform back across and drop down to the lower ledge. Make a portal of the other color on one of the walls here, then press the small red button and hop through the portal. Pick up the cube and take it with you through the wall panel. Put it on the red button and make a portal on the other side of the room where the chamber lock opened. Make a portal of the other color on the four-dot ledge to the left, then drop onto the four-dot ledge, and fall through the portal to reach chamber lock.
There is another area beyond. Drop down from the ledge and make a portal on the piece of floor in the pit in front of you. Then look down into the deep pit to your left and make a portal of the other color there. Drop through and you will be launched up from the small piece of floor in the pit.
As you fly up, shoot a portal of the other color on the ledge that you can see over as you reach the top of your jump, such that the portal that you just came through will still be there for you to fall through. Then, alternating the color, do the same for the higher ledges that you can see. Be careful not to touch the directional keys at all, to ensure that you continue falling through the portals below you. Eventually, instead of a flat ledge, you'll see an angled wall piece. Shoot a portal onto that to be dumped into the hallway where you can reach the elevator to the next text chamber.