Start the chamber by going to the left alcove next to the Buttons.
Place a portal onto the ceiling surface.
Place an another portal beneath you and while falling, place the portal that you've placed o the ceiling onto the sliver of a white part of the wall above the door. You'll be launched to the Cube. (don't be worried of you take damage, you won't die!)
Place a portal through the square-shaped hole in the wall.
Jump down to the Buttons with the Cube you've just acquired and place it on one of the Buttons.
Place an another portal on a nearby wall and walk onto the Light Bridge.
Teleport onto the upper half of the floor, where you'll encounter a Electricity Field.
Wait for the right time (when it's turned off) and walk over it.
Slide to the right into a small hatch (an arrow is pointing to the right spot).