I use CMC (Tylose) mixed with my fondant for the best results. It's all been said above but thought I'd add my tuppence worth. I've also broken a set of cutters because I smacked them too hard onto my worktop. I also roll the paste thinly and dip the cutter into cornflour before I use it. I leave the paste rolled out for a few seconds before I cut it so that it starts to dry a little. It makes flipping the letters out much easier. The more you use these you'll find your own way with them.