Remove the battery.

If the battery adhesive is still sticky, you can reuse it. Otherwise, remove all the old battery adhesive.

If your new battery has adhesive pre-installed, remove the plastic liner to expose the adhesive. Otherwise, apply strips of thin, double-sided tape to the battery well (not directly to the battery) where the old adhesive was.

Firmly press the battery into place, making sure the cables are near the rear of the case.