Go up the stairs and through the door. When you hop down, an orange portal will appear behind you. There are three rooms, one containing a cube, one containing a red button, and one containing the exit. Each room has a button next to it that you can press.

First, press the button next to the room that contains the cube. This will create a blue portal in the cube room. Go through the orange portal, take the cube, then go back out to the main room.

Now press the button next to the room with the red button to create a blue portal in that room. Take the cube through the orange portal and put the cube on the red button.

Leave the cube on the red button and go back out to the main room. Press the button next to the room with the exit door. Then go through the orange portal and into the room with the exit door. Go through and into the elevator down the stairs.