Peel up and remove the insulating sticker covering the battery board, on the edge of the logic board nearest the battery.

If the cover doesn't peel up easily, apply mild heat with an iOpener, hair dryer, or heat gun to soften the adhesive underneath, and try again.

If the cover doesn't seat securely during reassembly, try rotating it 180 degrees. It must be oriented correctly in order for the adhesive pads underneath to contact the board screws.