The first thing I would figure out is what your budget is and how "good" of a helmet you're trying to build.
I've considered scratch building one in the past but after weighing the options I decided to save up and buy a helmet kit. It's totally worth it in my opinion.
While there are so many great templates and resources for scratch building, it does take some time and materials but mostly time. I figured between the time it takes to research, fabricate, fiberglass, sand, etc. and the cost of materials the kit is your best bet.
And unless you plan on scratch building EVERYTHING you still are going to have to buy certain parts like some of the greeblies, visor, and range finder bits.
I'm not trying to dissuade you or anybody trying to scratch build pieces, it's just that there are a lot of options that have to be considered.
Hopefully this helps!