Remove the lower case.

To reinstall the lower case:

Lay it down and align the sliding clips with the back edge of the MacBook. Press down on the lower case and slide it toward the back edge to engage the clips.

When one side is engaged, it may push the other out of alignment. Check both sides as you push.

Once the back corners of the lower case are secured and flush with the frame, press down along the middle of the lower case to engage the four remaining clips.

You'll hear and feel each clip snap into place.