What is the best way to make a gumpaste letter or number stand up on the top of a cake?
I like to use a 50/50 gumpaste/fondant mix or fondant with some Tylose. I just form the letters-cutting them out or shaping them from fondant rolls-and push lollipop sticks or thin wooden craft sticks up through an upright portion. I lay them flat until they are dry/hard, flipping them over occasionally so both sides dry evenly.
Hope this helps.
I also slide a couple toothpicks into it before drying, then it has more to anchor to the cake instead of just balancing it on top.