The adhesive securing the screen to the back cover is very strong and requires a lot of heat before you can create an initial gap.

Note the location of the adhesive we're softening on this step: long strips secure each short edge of the screen.

Apply a heated iOpener to the left edge of the screen for three minutes.

Alternatively, use a heat gun or hair dryer to soften the adhesive. If you're reusing your screen, be careful not to overheat it. Heat it enough so you can barely touch it comfortably.