Go down the stairs to the main testing area. There are a couple of gel tubes here. They both drip gel occasionally. The rest of the testing area is wide open, and at first it's hard to tell what you're supposed to do. First of all, there's a big red button in one section, and there's a cube in another. You should get that cube and put it on the red button. Here's how.
First, look at the area with the big red button. There is a white floor near the red button. Put a portal there, then put a portal of the other color under the blue gel drip.
Now, look beyond the area with the gel tubes. There's an area with red arrows on the floor. The arrows are pointing toward a wall that has an angled wall on top. Put a portal on that angled wall. Now put a portal of the other color under the blue gel drip.
Now put a portal on the part of the wall below the angled wall, and put a portal of the other color under the orange gel drip. This will make a streak of red gel on the floor with the red arrows on it.
Now use portals to go where you put the orange gel. Put a portal on the wall at the end of the orange gel, then put a portal of the other color on the angled wall above that. Go to the end of the red arrows, then run toward the portal. When you come out of the angled wall, you should bounce off of the blue gel.
You should now be on the platform with the cube. Pick it up and drop down to the area below this platform. Put down the cube and look for the area where there is the red button (you can't see it from here) and a greenish pipe-like thing. Put a portal below that greenish pipe, then make a portal of the other color on a wall near you. Take the cube through. Put the cube onto the red button. This makes the ceiling thing rotate.
Note that the ceiling that you rotated should have blue gel on it. If not, you have take the cube off of the red button, go back to the area near the gel tubes, put a portal on the floor below the rotating ceiling and another of the other color under the blue gel drip. Now you can go back to the red button and put the cube back on.
Way off in the distance, across from the exit elevator, there is a white wall. If you put a portal there and go through, you find a ramp with a red arrow on it. Your goal now is to shoot some orange gel through a portal so it coats that ramp. Use portals to go back over to the orange and blue gel drips. Then put a portal on the wall facing that ramp (remember, it's across from the exit elevator). Put a portal of the other color under the orange gel. This coats the ramp with orange gel.
Finally, go to the area near the gel tubes where you made a long streak of orange gel on the floor. Use portals to go to that area, then make a portal on the wall facing that ramp that you put orange gel on a moment ago. Put a portal of the other color at the end of the big streak of orange gel that you should now be standing on. Run from the far end toward the portal at the end of the orange gel. You should run over the orange ramp, bounce off of the blue gel on the rotated ceiling, and end up at the elevator. Go in.