I've been wanting to learn how to do these but wanted to know the best cutter to use. Doing a wedding cake soon and she wanted two pretty large flowers, one on each tier.
In the Wilton Gumpaste class we used heart cutters for fantasy flowers. I'm not sure if that's the best but it may be a start for you. Good luck!
Edna De La Cruz has a set of fantasy flowers on her web store, I used them for one of my cakes
The ones I made for my black and white cake ( http://cakecentral.com/gallery/2296488/black-white-ruffled-all-over ) are my favorite (so far ) and I used peony cutters for those. I do think you can make a fantasy flower our of just about any cutter, though.