To install new rear glass:

Use tweezers to peel away any remaining adhesive from the phone's chassis.

Clean the adhesion areas with high concentration isopropyl alcohol (at least 90%) and a lint-free cloth. Swipe in one direction only, not back and forth. This will help prep the surface for the new adhesive.

Peel the adhesive backing off of the new rear glass, carefully line up one edge of the glass against the phone chassis, and firmly press the glass onto the phone.

If you're reinstalling the old rear glass, or using rear glass without adhesive pre-installed, follow this guide.