When you first enter the room, it's not ready yet. Wait for Wheatley to smash part of the room into the part you're standing in. After that, the light funnel will activate.
Put a portal on the wall where the light funnel is flowing. Then put a portal of the other color on the wall to the right of the exit door. Walk into the funnel to be sent to the other side of the room.
There is a wall that passes under the cube chute. It's to the left of the ledge where you're standing now. It's the section of wall closest to your ledge. Make the light funnel come out of that wall, then press the small red button to make a frankencube fall into the funnel. Afterward, make the funnel come out of the wall across from your ledge, on the right, such that the funnel will come out where the cube is being pushed against the wall. This will make the cube come to you.
Make a funnel come out next to the green target that is painted on the wall across from your ledge, on the left. Pick up the cube and step on the catapult on the side of your ledge. If you placed the funnel portal correctly, you will get caught in the funnel. From there, you can just walk to the first part of the test chamber, where the exit door is.
Now put a portal in the checkerboard floor to make the funnel come out of it. Pick up the frankencube and walk into the funnel. Let go of the frankencube, then make a portal on the wall across from the big red button. You and the frankencube will fall into the funnel below and be pushed toward the big red button. Get out onto the ledge with the exit door. When the frankencube opens the exit, go through.