If you are re-using your existing display, you must also remove the old adhesive from the back of the display.
Peel off the adhesive strips using tweezers or your fingers. Start at the bottom and peel upward towards the top of the device.
After all the old adhesive is removed, carefully clean the surface with 90% (or greater) isopropyl alcohol and a lint-free cloth. Wipe in one direction, not back and forth.