Your first call to action is to set a portal on the single square wall panel directly above the left-hand side of the light bridge in the top-left corner of the chamber, now place the entrance portal on the wall directly ahead of you.
Step through and you’ll fall onto the light bridge, note what colour of portal you used as your exit portal (the one leading to the light bridge) and reset it at the wall directly where the light bridge stops.
This will take you back to the start of the room. Halfway across this central walkway look to the wall on your immediate left. You should spot a wall panel made up of really small squares, fire your entrance portal nearer the top of it.
If done right, you should notice the bridge stretch right across the room, press the red release button nearby and a Companion Cube will drop down and should land safely on the bridge.
Carry the Cube over the acid and set it so it’s half on and half off the right-hand side of the light bridge.
Turn around and edge yourself over the bridge (to your left) and fire your exit portal so a bridge appears almost directly below you.
As you drop down, immediately spin to your left and catch the Cube mid-flight, you can now simply walk over to the button, drop the Cube on it and leave via the exit!