If your iPhone has a cracked rear glass, keep further breakage contained and prevent bodily harm during your repair by taping over the glass.

Lay overlapping strips of clear packing tape over the iPhone's rear glass until the whole face is covered.

Wear safety glasses to protect your eyes from any glass shaken free during the repair.

If you can't get the suction cup to stick in the next few steps, fold a strong piece of tape (such as duct tape) into a handle and lift the rear glass with that instead.

If all else fails, you can superglue the suction cup to the rear glass.