Look for a catwalk high up above. There is a door at one end. Use a portal above the door to reach that catwalk. Go to the other end of it, up some stairs. Turn around and you will see an office on the far wall to the left. Across from the office, there is a white wall up above. Put a portal there, then put a portal on the floor that you can fall into. Fall into it.
Go over to the poster when GLaDOS/PotatOS asks you to. Then press the big red button nearby. The huge ceiling hatch will fall open and a small platform will come down from above. Jump down from the office and go to the platform. Get onto it and it will automatically go up.
Above, step off of the platform and go up the nearby stairs. Go through the emancipation grill and get into the elevator.