Chapter 2 - Chamber 7
Use the third cube to block the laser and call the elevator. Now use the white platforms to bring the box to the pad, and shown in the video. Reach the exit zone, and take the hint from Glados. Shoot a Portal on any surface in the hallway, and take the cube through the other one, unlocking the achievement!
You can also get this achievement in Challenge mode, playing "The Cold Boot - Pit Flings".