The easiest way is to get a form, to make sure the armor is big enough. Ideally this would be a large fiberglass copy of a Predator with no armor, but you could improvise numerous ways. Then do you sculpt out of a medium or hard plasteline like Chavant NSP. Make a silicone jacket mold. Then cast up the pieces in bondo resin backed with a few layers of fiberglass. Its light and strong and easy enough to paint.
I hope that helps. I can interject more detail if you like.
OK, simply put, you need a form to start with. I reread this. Are you doing this for action figures? Regardless, the process is about the same. Pick a form that is the same size as the piece you want to attach the armor to. It is best not to sculpt on the actual figure or costume, but you do need to make sure the finished armor will fit. Now that you have a form, do the sculpt in a medium/hard plasteline, like Monster Makers Clay or Chavant. The smaller the piece, the harder the clay. This is to ensure precise detail. When finished, you need to seal the clay in aerosol acrylic lacquer, and then possibly a coat of dulling spray, unless you need a glass smooth finish. Do a silicone brush up mold of the sculpture, and back this with plaster bandaged. When everything is set, remove the new mold. Now you can brush in a layer of Bondo Resin (a custom gel coat we mix in the biz, basically 65% autobody filler catalyzed with the cream hardener, and 35% polyester resin catalyzed with MEKP). When this layer is dry, do 2-3 layers of 1.5 oz chopped strand fiberglass matte. I like to follow this with a layer of light weight fiberglass cloth for a nice finished surface. Then demold this new part and sand and trim as necessary, then paint.