Step 2: Lift up screen
Now you make good use of the suction cup or the thin metal spatula in order to lift the screen out of the frame. Which of the two you will use depends largely on the state of your screen. If it's still mostly uncracked, the suction cup will work just fine.
If, however, it is utterly destroyed, this is likely not to work. In this case you should grab the metal plectrum from the repair kit. Alternatively, you could try creating a flat surface for the suction cup to hold on to by applying some tape to the screen.
Drive the plectrum between frame and screen. Be careful not to insert it too far into the device in the area around the home button. Slowly lift up the screen, together with the inner metal frame it is attached to inside the device.
Warning: Open it very slowly at first and be careful not to rip the cable connecting the home button.