This is the first light bridge test. You’ll see the light bridge when you enter the chamber. What you need to do is place a portal on the wall where the bridge hits it. And then place one across the way where the door is. (Strictly speaking you don’t need the bridge at this point but you might as well do it now so you don't have to move that portal for the rest of the chamber.) Go through the portal and you’ll be on the other side with a small button. Press it and it will start dropping cubes. To get to them you’re going to have to work your way around a corner with the bridges.
In the center chasm, there will be an area of wall you can place a portal on. If you’re on the side with the button it will be on your right, and if you’re on the starting side it will be on your left.
Get back on the starting side and look to your right. There will be a glass enclosed area you can see the cubes falling in and next to that, a bit closer to you, will be an alcove. Place a portal somewhere on the left wall (while facing the exit) so that the bridge extends into this alcove. Try to place it higher rather than lower as you’ll need to change the portal location twice more and you’ll want to have some room to fall before hitting the water.
Walk across the light bridge until you reach the back wall of the alcove. Now turn to your left and you will see another section of wall you can place a portal on. Taking care to make sure you move your exit portal and to make sure the bridge will extend below where you’re currently standing place the portal on that wall. Follow the bridge towards that wall and on your right will be another section. Do the same thing to move the portal and extend the bridge below the Aperture Science Weighted Storage Cube vent and grab one as it comes down. Bring it with you through your portal, portal over to the other side to place on the big button and head for the elevator to get your achievement.