What they said, and I would add...cover your visor with blue painters tape to protect it from scratches, make a line down the center, place your visor cut out area of the helmet on the line and trace, create a visor that is approximately 1/4" larger all the way around your traced area and as symmetrical as possible.
If your visor cut out piece is trashed, then trim the shield to accommodate the cheeks and place the visor in the helmet and trace the cut out area on the shield using the center line. Then create your 1/4" larger visor.
