Use this guide to correctly apply replacement battery adhesive strips in many late-model iPhones. Any time you remove the battery, these strips must be replaced. The strips are easily damaged if not applied correctly, so you may want to have a spare set of strips on hand if following this guide for the first time.
The larger iPhone "Plus" models use three strips instead of two, but the procedure is the same.
Before you begin, you should have removed the battery by following a guide like the ones linked below. Note: iPhone 5 and earlier models do not use any adhesive strips.