Check out this fabulous tutorial I found on the Scrapbook & Cards Today blog and also featured in the brand new CardMaps Today Vol 2.
Trim a 12" piece of .75" ribbon. Run a gathering stitch down one long edge using a thin twine, leaving 4" tails on each end.
Gently pull on the twine tails to gather the ribbon. The tighter you pull, the smaller the rosette will be.
When you are happy with the shape of the rosette, tie the twine into a knot and trim off the tails.
Insert a decorative brad into the centre of the rosette and adhere it to your project! Voila!