Your first port of call is the red plunger on the right-hand side in the middle of the room. This’ll activate the blue portal in here, so turn back around and walk into the orange portal. You’ll reappear in the section where the Companion Cube is resting.
This’ll open up the exit, so double-back on yourself, press the third - and final - red plunger in the room and make your way back through the portal one final time. Step into the elevator to finish the level.