Pre-Cut Back Battery Door Cover Adhesive Double Sided Tape Glue Replacement for Samsung Galaxy S21 5G SM-G991U G991U1 G991 (2-Pack) - Easy DIY Repair for Cracked or Loose Phone Back Cover

Everybody says that it is easy to install, but it was not obvious to me, so here is what I did. I positioned the adhesive with the protective plastic films against the body of the phone (in my case the thicker green plastic was in contact with the phone); then I peeled the larger clear protective plastic to expose the adhesive that faces the cover. I pushed the cover against the phone with the green plastic film in place. I pulled out again the cover and made sure the adhesive was correctly stuck on the cover by pushing around the cover on the green plastic film. Finally, I pulled the protective green plastic and put the cover back on the phone. While it looks perfect, I hope it lasts (If it doesn't I will be back).