If I were you, I would look on YouTube. Before I started my first suit I pretty much lived on YouTube looking at foam builds. I would recommend looking at Punished Props, he is completely EVA foam but not completely halo stuff. It's a good reference for how to get certain things done when working with foam that you can apply to your build. Also check out Evil Ted Smith. He has these "tricks of the trade" videos that lay out pretty much everything you will need to know to get started.
If your heart is set on building a Spartan I understand, but also if you look at Andrew DFT on YouTube he has cheap templates you can buy from Etsy and he has a video of every step of every piece of armor. For sure I know there is a marine and he just did an ODST. Its a great starting point in my opinion.
Good Luck!