What is your budget? The easiest way is to cast the gems out of resin and then glue it into preformed metal rings which can be bought at jewelery stores or sometimes you can use plastic cute pieces though they will be a little thicker. However this adds up in cost.
I made my gems out of sculpey- basically a red slightly concaved sircle with a yellow snake wrapped around xD
and hot-glued brooch backings that you can find in the jewelry section of joanns, to the backs of 2 smaller ones (hands) and one larger one (cape) the hand ones have 2 brooch backs on each and the larger one has 3
for her belt I spray painted a normal belt yellow, but you could try to find a real yellow belt and hotglued on the remaining gems
I second the resin casting. It's how I make my gems and it's not nearly hard as it seems. Just follow the directions carefully. As for the backing, you could probably get away with some sealed foam discs that are painted and backed with like card stock/thin card board or something to keep it rigid.