If reusing the rear glass panel during reassembly:

Use tweezers to peel away any remaining adhesive from both the rear panel and 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.

Be sure to turn on your phone and test your repair before installing new adhesive and resealing the phone.

Peel away the replacement adhesive's liner, align it with the back of the phone, and press it into place.

Remove any remaining liners, and replace the rear glass panel.

After installing the rear panel, apply strong, steady pressure to your phone for several minutes to help the adhesive form a good bond, such as by placing it under a stack of heavy books.

You can also reinstall the rear panel without replacing the adhesive. Remove any large chunks of adhesive that might prevent the panel from sitting down flush. After installation, heat the panel and apply pressure to secure it. The phone won't be waterproof, but the residual adhesive is usually more than strong enough to hold.