Once you figure them out you will love them! Edna has some great tutorials on You Tube using them but here is what I do...
You need to use Gumpaste NOT fondant. Fondant is too sticky and they just stick to the cutters.
Roll your gumpaste very thin and I allow mine to set for a few minutes to dry a bit. Cut your gumpaste in small pieces the size of the letter you want... press the letter into the gumpaste and lift it. Sometimes it sticks to the tappit and I just use the sharp pointy tool... I think it is a stiletto... to go around the edge and then it comes right out.
When I first started using them I was ready to trash them... .but now I love them! Don't give up... letting the gumpaste set and dry is the key.