Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:4851431.

I printed the Broken Nerd's Mandalorian helmet. https://www.thingiverse.com/thing:4401649

Any video I watched on visor making told me to hold a piece of paper in there and poke a pencil in to trace the slots while trying not to puncture the paper and the flesh of your hand.

I decided to plaster the inside of the helmet with duct tape, draw it properly and then flatten and build a stencil.

The stencil can then be resized to whatever ratio you use when printing your own helmet.