Go straight ahead into the testing chamber. There is a "thank you" sign on a wall. Above that there's a white ceiling. Put a portal on that ceiling, then make a portal of the other color that you can walk into. Go through.
Look at the floor and notice that there is a dark line going toward the ramp. There is a white wall at the end of the dark line, facing the ramp. Put a portal onto that wall. Now look at the ceiling. There is a tube that has orange coloring on the end. Put a portal of the other color on the floor below that tube. Now press the red button. This will make a bunch of orange gel coat the dark line on the floor.
Go down to the floor and walk along the line of orange gel. This type of gel makes you move much more quickly. Walking on the gel and up the ramp should propel you to the ledge on the other side.
Now you have to make some gel come out to the area that you just landed in, but you have to be careful in order to be able to reach the red button. Look back to the area where you pressed the button. There should be a portal on the ceiling there. Create a portal of the other color on the wall that has the yellow and black symbol in the center. Go through. You should land near the red button. Now make note of the portal color on the ceiling above you. Put a portal of that color on the spot where the orange gel will fall. Press the red button.
Now create a portal on a wall of the same color as the portal that you put on the floor below the gel tube. Go through that portal and run along the orange gel to reach the next area.
Look to where the red arrows are pointing. There is a cube behind a glass wall. Across from that glass wall is a white wall that is not blocked by glass. Create a portal behind the glass, then a portal of the other color on the white wall across from the glass. Go through to get the cube.
Now look back to the area that you came from earlier by running on the gel. Put a portal on the white wall over there. Put a portal of the other color on the unblocked white wall near you. Take the cube through.
Look at the testing area where you pressed the red button to trigger the orange gel. Put a portal on a wall over there, then put a portal of the other color on the wall that you just came out of. Take the cube through.
Put the cube on the big red button. This makes the ramp go up more. Run along the orange gel and up the ramp. This sends you up to the higher ledge.
Look to the right of the exit door. Put a portal on the white floor there. Now put a portal on the ceiling near the red button that activates the orange gel. Drop through the floor portal. Now look at the ceiling portal that you came out of. Put a portal of that color on the floor where the orange gel will fall. Now press the red button. This should coat the wall near the exit door with orange gel.
Look at the color of the portal that you put below the orange gel. Put a portal of that color on the wall where the nearby line of orange gel is going toward (the wall that faces the ramp). Go down there and run from the ramp through the portal. You will end up in front of the red button near the ceiling. Press it to open the exit door. Go through.
Now when you look to your left, you will see a gate, beyond which you can see a white wall. If you want to unlock an achievement, put a portal on that white wall, then put a portal of the other color on the wall near the second emancipation grill. Go through and look around in the office. There's a portrait in there that GLaDOS will react to. This unlocks the achievement.
Now go through the second emancipation grid to the next test chamber.