Take your charm and glue a jump ring to the top or center of it. I used a broken hair clip for mine, but you can use anything you want. If you can't find anything lying around, try making your own charms from beads!
Connect another jump ring and a jewelery clasp to the charm.
Clip the charm onto the center of the chain. Then add a jump ring & jewelery clasp to the ends of the chain and connect them.
Repeat steps 1 & 2 to make several charms. Change them whenever you'd like, or wear them all at once!