Select a barrette that's flat on top.
Fold a piece of felt in half. Starting at the fold, draw half of a butterfly. The length of your butterfly along the fold should be at least as long as the barrette.
Leaving the felt folded, cut along the line you drew. Open the butterfly.
Cut out details from felt. Glue them on the butterfly, using tacky craft glue. Let the glue dry.
Open the barrette, and glue the butterfly on top of it. Cut out a square of felt, and glue it to the underside of the barrette, as shown.