Ok, I didn't take pictures of me making the costume, but I did do it recently and in a short amount of time, like less than a day, so I can give you guys a few tips and the ending result :)
Step 1: Tools and Equipment
Firstly, when it comes to building, you always need the right tools to get it done.
You would need images as a guide line a straight edge (ruler), exacto knife and or scissors (with parental guidance or permission of course), a sharpened pencil or pen cardboard and of course DUCT TAPE BRAND SCOTCH TAPE.
(unfortunately, since I finished the costume before I knew about this duct tape competition, I threw out my tape since I finished it.)
Step 2: Starting on Cutting, Shaping, Measuring Etc.. LOL
Ok, when ever I begin making my helmet, I always start with the face plate, or the face area. There are many ways of measuring your face and getting it right, like using measuring tape, or simply putting it against your face and marking it with a pencil. It will be more symmetrical by measuring first though.
Step 3: Advice
Remember, when you do it, MAKE IT SYMMETRICAL. If you end up wanting to fibre glass it then detail it with bondo, you'll have difficulties, trust me, I faced that first hand.
And stay modivated, or you will never get it done and it will take forever till its finished.
Message me if you need help. I can make a new one in the future and post it here iwth step by step info.
Step 4: There. LOL
I didn't full complete it, because once halo reach came out, I liked the suits in the new game alot more than halo 3. But as you can see the helmet and body could be fibre glassed and detailed with bondo if I decide to. But for now I'll focus on my current project that I'm bringing to comic con in Winnipeg, go to my page and check out my progress (I havent updated it in a month though LOL)
So if you are planning on building something like this and need advice, than message me anytime :D
Completed, it cost me under 15 bucks, mostly because of the tape. But the cardboard was used from my old moving boxes.