Here is a walk through of how I created my helmet "Stanley" for my Sizer cosplay.
A few tips when working with spray foam:
1. Work in small bits, if you spray too much at a time, it will become hollow or may not dry on the inside.
2. You can spritz the foam with water to speed up the hardening phase. Do this between layers of foam to ensure you don't get a hollowed out mess.
3. Spray foam sticks to everything, make sure you wear protection and do it in a place like a garage or outside.
4. Spray foam can be removed from skin while it is still wet with acetone. If you get any on you remove it immediately.
5. You can buy in expensive disposable razor blades at your local hardware store. When they become dull use a new blade, dull blades are actually more dangerous to use then sharp ones.
Hope this gives you some ideas of how to tackle your next armor project.