Go up the stairs and then wait for GlaDOS to move the panels out of the way. Continue on ahead, dropping to the lower level to go through the entrance to the test chamber.
There is some sludgy liquid below, a laser to the right, and a cube chute across from the entrance that you just came through. In addition, there is a laser receiver on the wall to the right of the entrance.
The first step is to get a laser to shoot into the laser receiver. To do this, just make a portal on the wall across from the laser receiver, then make a portal of the other color on the wall where the laser is shooting. This will cause a platform to move side to side on the wall across the pool of liquid.
Remember which color portal is across from the laser receiver. When the platform is directly across from the entrance, shoot a portal of the opposite color into the cube chute area to make the laser stop hitting the receiver and make the platform stop moving.
If you did it correctly, you can now walk through the portal that the laser had been coming out of a moment ago, and walk into the cube chute area. Press the red button, pick up the cube that falls, then carry it onto the platform that had been going side to side a moment ago. Put the cube behind you so that it can act as a shield to protect you from the laser shooting from the wall, because you will be crossing the path of that laser in a moment.
Now you need to put a portal on the wall that the laser is hitting, but make sure that it's the opposite color from the portal that is currently facing the laser receiver.
The platform should start moving. Once the cube (which you should have placed behind you) blocks the path of the laser, the platform will stop moving. Walk past the laser and pick up the cube. When you get to the other side of the room, you can carry the cube to the exit area, put it on the red button, and exit to the elevator.