Peel the battery's adhesive sticker off the midframe.

This step is mandatory if you are replacing the screen. However it is highly recommended to use a fresh adhesive sticker as well, when installing a new battery.

Secure the new battery with pre-cut adhesive or double-sided adhesive tape. In order to position it correctly, apply the new adhesive into the phone at the places where the old adhesive was located, not directly onto the battery. Press the new battery firmly into place.