Ill try to help you out in a way that I am making my armor. Dont worry I already tested everything and it all works. Also this is a very small tutorial just because im sure you know what to do with the fiberglass and all.
Sculpt your helmet out of clay. It will be hard, trust me, I know. You want to detail it and make it proportionate. My suggestion would be to get a mold of your held from plaster bandages, and then fill it with plaster. After you are done with that, You can start forming the clay around it. When done with the helmet, What I would do is apply 20 layers of latex ( Why latex, you ask? well its so the fiberglass doesnt stick to the clay. ) Then apply your fiberglass after the latex has had enough time to cure. When everything is dry remove the plaster cast of your head along with the clay.
Repeat those steps for the armor.
Other people on here though might have better ways than me or similar. This however is the best way for me to make armor.