Optactical has a how to guide on their website.
My weapon painting efforts got started with .mil weapons, and I've mostly just duplicated that on the couple long guns of my own I've painted -- krylon, use some locally procured foliage as a negative-stencil (or go hogwild and buy some of that raffia stuff in the crafts section of walmart for a couple bucks), makes for a solid end result. And if you get scrapes, scratches, and such that you don't feel add to the character, you can always just give it a two second touch up and you're good to go again.
Only stuff I ever taped off were magwells, exposed bolts or other internals (not an issue on an AR, just close the dust cover), iron sights, and serial numbers (last optional, but seemed like a good idea to me).