I need to make a #21 to go on top of my daughters birthday cake....
how do I make it, and what should I make it out of..
You can hand roll a snake from gum paste with tapered ends and shape it into the number you need. insert a toothpick part way into the bottom and let it dry. Insert into the cake after decorating.
I always use cookie cutters but they are not that big. If you want to make a big 21 I think you can make it out of gumpaste, but I've never done one.
Maybe the experts can help you soon.
Sadsmile is right, that's the easiest way to do it. I use a plastic lollipop stick if it's a heavy piece, because it's sturdier than a toothpick. It would look really neat to make some bow loops and then have the 21 coming out of the top of the bow.
You can certainly do it as sadsmile said.
Another option is to use a chocolate mold and make them similar to suckers.
You can also print out a font you like and use it as a template to cut flat (thick) pieces of gumpaste/fondant. If you do, then you can insert a sucker stick/toothpick before it dries. If you waited too long and it gets dry, you could also use chocolate to attach a sucker stick to the back.
Royal icing is another option, but I've never done it.