Up ahead, you'll have to wait a moment for the platforms to get out of the way of the entrance door.

In the testing chamber, before stepping onto the aerial faith plate near the entrance, walk to the left of the entrance door as far as you can go. Turn around and look in the direction of the entrance door. To your left, you should be able to see a couple of angled portal walls. Put a portal onto each of these angled walls. Now you can go back and step on the aerial faith plate.

If you placed the portals correctly as described above, you will be sent to the side of the room with the exit door. There is a small red button here that will make a cube fall onto the aerial faith plate that you first stepped on. Before you press the button, notice that there is a big red button across the way, and there is a portal wall on the ceiling above it.

As an aside, you can get the Final Transmission achievement in this room. Press the small red button and some junk will fall from the chute near the entrance. If you left a portal on each of the angled walls, this junk will come over to your area. Pick up the clock radio and take it back to the entrance. Then go to the end of the ledge so you can see the two angled walls. Put a portal on the rightmost angled wall, then put a portal on the far right of the big strip of white wall above the angled walls. Pick up the clock radio and get on the aerial faith plate. You should unlock the achievement. Once you do, go back to where you can press the small red button that made trash fall down earlier.

Check the colors of the portals on the angled walls. You want to leave the portal on the angled wall on the right, so shoot a portal of the opposite color onto the ceiling above the big red button. Now press the small red button, and if the portals are positioned correctly, the cube will fly through the portal on the angled wall and fall out of the portal on the ceiling above the big red button.

A bunch of junk will come out of the chute the first time (if you haven't made it come out already), so if it doesn't successfully land on the button, try again a few times. If it still doesn't work, put the portal back onto the angled piece of wall that no longer has a portal on it, then press the button to make a cube fly over to this area. It will probably fall into the pit to the side, but you can easily use portals to get it out of there, then put a portal back on the ceiling above the big red button, and use another portal near you to put the cube through it so it drops onto the red button.

Go into the exit that opens once the cube is on the big red button, and go into the elevator after GlaDOS calls you smelly garbage.