Push down upper PCB, reinsert and tighten the 2 black screws, if you removed all 4 during disassembly, remember to put the lens and iris back under the upper PCB before pushing it down and tightening all 4 black screws.

Reinstall the narrow brown ribbon, starting with the retaining clip pulled loose, there is a darker brown reinforcing strip on the non contact side of the ribbon, the edge of this strip can be pushed on with your fingernail or a small flat blade screw driver or tweezers to push the cable in, the clip will push in as the cable is pushed in.

Reinstall the white ribbon into both the upper PCB and the PCB attached to the top cover and rotate the black clips into the flat locked position.

Hook the thin edge of the top cover over the thin edge of the main body of the mouse.

Start closing the case by slipping the plastic retaining pin on the left side into its hole first then push down the right side clip over the metal clip.

Reinstall the battery and close the battery cover.