To use a tappit cutter:
Simply roll out some gumpaste very thin using cornstarch to keep it from sticking to your surface. Cut the gumpaste into strips and lay the desired letter across the gumpaste (so it looks like a cross). Press firmly against the backside of the cutter, rocking it back and forth to ensure a clean cut all around. If the letter or number remains in the cutter, simply tap it loose. If it is stubborn, you may need to coax it out by pushing from the back with a small gumpaste tool. The cut out letters can be positioned directly onto your cake at this point, or dried firm so they can be free standing.