Remove the lower case.

During reassembly:

Set the lower case in place and align the sliding tabs with the screw heads they slide over. Press down and slide the lower case toward the back edge to engage the tabs—it'll stop sliding as the tabs engage.

When one side is engaged, it may push the other out of alignment. Check both sides as you push, there should be no gap along the back edge.

Once the lower case is flush with the frame, press down firmly along the perimeter to engage the four snapping clips.

You'll hear and feel a distinct click as each clip snaps into place.