Chapter 2 Chamber 6. When you get to the button to release a cube when you first press it glados will drop a load of crap out of it including a turret needed for this trophy and a radio needed for this-
move both your portals of the slopes at the far end of the chamber before pressing the button and the items dispensed will end up on the ledge where the 2 sloped light grey walls are. Go collect the turret and take it back to the start of the level via a portal then throw it on the aerial faith plate. Trophy unlocked
There is also one other trophy on this level-
The Drop Box trophy in Portal 2 worth 42 pointsPlace a cube on a button without touching the cube