Use tweezers to peel away any remaining adhesive from both the back cover 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.
After installing the rear cover, 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.