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 bottom edge with your suction cup, opening a slight gap between the back cover and the frame.

This may require a significant amount of force, but you only need to open a very slight gap with the suction cup to insert your tool. If you have trouble, apply more heat to further soften the adhesive, and try again. The adhesive cools very fast, so you may need to heat it repeatedly.

Insert an opening pick in the gap and slide it to the bottom right corner.