iFixit

Before beginning, unplug your iMac and lay it on a soft surface as shown.

Stick a suction cup near each of the two top corners of the glass panel.

To attach the suction cups we sell, first position the suction cup with the movable handle parallel to the face of the glass panel. While lightly holding the suction cup against the glass, raise the movable handle until it is parallel with the other handle.

If your suction cups refuse to stick, try cleaning both the glass panel and the suction cup with a mild solvent.