If the phone's rear glass is cracked, the suction cup may not stick. Try lifting it with strong tape, or superglue the suction cup in place and allow it to cure so you can proceed.

Press a suction cup onto the back cover.

Lift the back cover's right edge with your suction cup, opening a slight gap between the back cover and the frame.

Insert the tip of an opening pick in the gap you created and slide it to the bottom right corner.