Step 8: Optical Sensor
Make sure your completed Mouse Mouse is dry first, or the skin may pull and warp as it dries!
Use your X-Acto knife or scalpel to carefully trim a hole for the optical sensor to peek through, then add just a touch of cyanoacrylate (tissue glue, aka superglue) to the edges of the skin. This will toughen the skin edge and fasten it to the plastic around the sensor.
Use a tool (I used a bent piece of wire) to apply the glue, and do so sparingly- you don't want to drop any on the optical sensor!