If the phone's rear glass is cracked, a suction cup may not stick very well. 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 rear glass close to the bottom edge of the phone.

Lift the bottom edge with the suction cup, opening a slight gap between the rear glass and the frame.

Insert an opening pick in the gap and slide it to the bottom right corner. Leave it there to prevent the adhesive from resealing.