Get a store to print out a vinyl, or draw it onto some painters tape, cut it out, then place the tape onto the helmet.
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For the skull on this helmet, I started with taking an ingame screenshot of the helmet, and sent it to my computer. I zoomed in until it was roughly the right size, and turned the brightness all the way up. I placed tape stuck together into a square over it, and traced the skull with a pen. Removed tape, and cut out with a knife. Then cut a square in a plastic shopping bag, placed the square over the hole, and put the helmet in the bag so it didn't get painted except for the decal. I know the computer part is risky, but you can use a phone or something. Just make sure you dont press hard and dont care if you do mess up your screen...