If you're using a single suction handle, apply it to the bottom edge of the phone, while avoiding the curved portion of the glass.
If your display is badly cracked, covering it with a layer of clear packing tape may allow the suction cup to adhere. Alternatively, very strong tape may be used instead of the suction cup. If all else fails, you can superglue the suction cup to the broken screen.
Pull up on the suction cup with firm, constant pressure to create a slight gap between the front panel and rear case.
Insert an opening pick into the gap.
The watertight adhesive holding the display in place is very strong; creating this initial gap takes a significant amount of force. If you're having a hard time opening a gap, apply more heat, and gently rock the screen up and down to weaken the adhesive until you create enough of a gap to insert your tool.
Open the iPhone by swinging the display up from the left side, like the back cover of a book.
Don't try to fully separate the display yet, as several fragile ribbon cables still connect it to the iPhone's logic board.
Lean the display against something to keep it propped up while you're working on the phone.
During reassembly, lay the display in position, align the clips along the top edge, and carefully press the top edge into place before snapping the rest of the display down. If it doesn't click easily into place, check the condition of the clips around the perimeter of the display and make sure they aren't bent.