I can help you. I started out with the belt or crotch piece cause it's the easiest. Then I went on to the chestplate so I wasn't just walking around in a green diaper. Then the biceps and forearms then the legs. I did the feet before I finished the gloves. It's really easy to make things outta cardboard as long as you have enough boxes. I wouldn't recommend you make a spartan helmet out of cardboard, because it would probably be the hardest. I wish I could show you pics of the development of my armor, but all I have is the finished product. Enjoy!
I started with the lower legs & the handguards, then went to the crotchpiece. The helmet is gonna be a b*tch & a half if you don't glue on pepakura designs to the cardboard. The helmet is the hardest so after that's finished you should have it easier (unless you have more than 1 head).