For one thing, stickies. Read all of them. Google Halo 1 or 2 depending on which armor you want. MKV was the armor worn in Halo CE. MKVI was worn in Halo 2. Halo 3 armor is on the way, at least for pepkura, aka the paper folding way.
Take all the pictures you can find of the helmet and study then and build your helmet that way. Ive never done cardboard so, im just guessing about the approach.
Stickies. Read em. Then read em again. There are a few guys on here that used cardboard and they look fantastic.
Hope this helps.