Step 6: Finishing the Bottom
In order to fill in the gaps, I marked the mirror with a sharpie, and used a glass cutter and large pliers to cut the squares to shape. Ideally one should use a proper breaker as the pliers I had sometimes shattered the glass, but it was what I had on hand.
Cutting such small squares is difficult. Have patience. You will get better at it by the end of the helmet! And be sure to reuse pieces. Often the discarded half of one piece was a perfect fit for another empty space on the helmet.