Remove the back cover.

This is a good point to power on your phone and test all functions before sealing it up. Be sure to power your phone back down completely before you continue working.

Remove any adhesive chunks with a pair of tweezers or your fingers. Use some high concentration (over 90%) isopropyl alcohol to wipe away any adhesive residue.

If you're using custom-cut adhesives, follow this guide.

If you're using double-sided tape, follow this guide.