If your display glass is cracked, 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 display until the whole face is covered.

If the broken glass makes it difficult to get a suction cup to stick in the next few steps, try folding a strong piece of tape (such as duct tape) into a handle and lifting the display with that instead.