Maintain your grip on the front of the mouse with your finger tips.

Grab the battery cover by its edges with the fingertips of your free hand.

Locate the following:

Battery cover insertion prongs.

Insertion prong holes.

Insert the battery cover by placing the insertion prongs into the holes near the middle of the mouse at an angle.

Once the prongs are aligned and fully inserted, lower and press the back end of the battery cover until it a snapping sound is heard. The cover should be secured in place.