Remove the battery.

If you're reusing the battery, the adhesive should be sticky enough to use again.

If the adhesive isn't sticky enough, or if your new battery doesn't come with adhesive, apply strips of thin, double-sided tape to the battery recess (not directly to the battery).

If your new battery has adhesive pre-installed, remove the plastic liner to expose the adhesive.

Make sure the cable is pointing towards the front of the shell (closest to the scroll wheel), and firmly press the battery into place.