go to smoothon.com
there you can find a lot of great resources to cast a helmet. I recommend doing a 1 part brush on silicone glove mold with a plastipaste shell. You will want to key the mold so it dosnt warp. Smoothon has a dvd video showing how to cast and have a help line as well. You will need a scale to measure out the good silicone so I would get a 5lb digital scale so you can measure in grams easily.