I have an 8 year old Son who loves Marvel and loves cosplaying. I'm not very good / experienced at this sort of thing, but I bought the toy mask (it is just a mask, it comes with a little blaster as well, and has holes where the eyes are, which looks terrible).
I used 11x17 craft foam, glued 3 or 4 sheets together, then put the mask on my Son's face and wrapped the foam around his head and marked it, then glued it to both sides of the mask. I then drew the design of the helmet on the foam and cut it down to size/shape. I slapped several layers of Elmer's glue / Sticky Glue / water on it to stiffen it up. Then primed and painted it.
For the eyes, I bought a $12 pair of sunglasses that had reddish lenses in them, popped the lenses out, drimmeled them down a bit and then affixed them to the inside of the mask.
Considering I've never done anything like this before, I was really happy with the results. My Wife also made the trench coat, based on a children's sewing pattern for a bath robe. She worked off of that to get the basic design and then altered it to our liking. Then made a shoulder belt out of leather-ish fabric, painted up a couple of those orange toy blasters, and wa-lah!
Hope this helps. I'm not on RPF all that often so feel free to PM me if you have any questions. Good luck! My Son looooooves this costume.
Oh, and we went to the GotG red carpet premiere in Hollywood, and James Gunn took one look at the costume, and immediately came over to talk to us for a couple of minutes. It was awesome!