Get a small square (or whatever shape) mirror, available at your local walmart in little baggies in all sizes :)
Choose your beads!
Transparent ones look the best. Marbles that are flat on one side also look great, these come in bags (near the fish bowl decor sometimes :P)
Use a hot glue gun to glue on the beads any way you like!
Something else you'll need are these small metal loops that come with necklace and bracelet clasps
Use the hot glue gun to glue one small one to the corner (or desired area) of the mirror so you can put it on a necklace
I usually link too together (seen below) so the charm is centered :)
Add a pretty chain and, well, you're done!