really the simplest way is pepakura, but in a way you have to have the most time for it. i recommend, start with a size tester of a helmet, just to get your pepping skills started, and then maybe a mid-def helmet. it will take a few days to finish, but slowly, and steady, it will come. this is nothing you can rush, because otherwise, you wont be happy with it
im building my war machine for 3 months now, and still have to finish a lot of pep work ( alltho i finished a batman sculpt, and had some university stuff and some moving to do, so with enough time, i could be further on by now, but thats besides the point)
also, be sure to use proper safety equipment. its so important, i cant even begin to tell you.
look for youtube vids, to help you on your buillding process, stealth(on this page) has a youtube channel (i think its called the stealth props) where he does a step by step tutorial from paper to finished product
hth
EoS