You'll want to try and support your helmet to prevent warping, so cut out card board shapes that fit both vertically and horizontally into you helmet, just cut them so they all fit together. Then you are ready to resin, you should be aware there are two types of resin, polyester (which is fiberglass resin) and epoxy resin, I personally used epoxy as I though it would be less hazardous - oh I was way wrong. For both types YOU NEED a respirator, no ifs or buts, trust me it's not good for you. You may be able to get away with epoxy resin on a very windy day but that's risky. Thin coats are best, I got away with a thin outside coat and a bit heavier inside coat and that was me. After this you can either use rondo ( to long to explain here, just look it up ) or fiberglass matting/cloth to reinforce the helmet. Then finally bondo, again look up tutorials, there is a guy on YouTube making an iron man helmet with excellent step by step videos on how to go from resin to fine detail.