This is what you'll need.
1. Tiny piece of fabric, whatever color you want, I picked this because it was the cheapest and ugliest I had i my fabric drawer (never waste nice things) and it sort of matches the coloring of the beads.
2. Obviously some of those hair clip thingies, you'll want to get ones with holes
3. Needle and thread
3. Beads or buttons or whatever you want to sew on
4. For extra hold, some textile glue.
Cut out a piece that is twice as wide and a bit longer than the barette.
Fold and glue together.
Fold in the edges so it doesn't look too home made, and while your at it, stick a bead on the needle and start sewing stuff on.
While you're still sewing on the edge, attach the fabric to the barette.
The you just keep on sewing beads on.
Once you've reached the other edge, you just fold it in and put a few stitches in it
(You don't really need to glue it but it keeps it from sliding and showing the metal.)