For the cupcakes, you want to use a recipe (or boxed mix) that bakes up into a nice dome, but if you end up with flat cupcakes, you can fake that dome with lots of frosting. No one will know the difference until they bite into them and then they'll thank you. I found that the most important key to success with fondant is to keep it in a sealed bag any time it's not being used. Fondant dries out quickly and will become difficult to work with (crack) if it loses too much moisture. If you are working slowly, your best bet is to cut and cover in batches so your pieces aren't sitting out while you make each cupcake. I found this video on how to cover a cupcake in fondant very helpful, as someone who had never done it before. When you insert your toothpicks, do so carefully and only through the head. Pressing too hard or into the body can cause the dried royal icing to crack. Instructions for the Marshmallow Stormtroopers can be found at Catch My Party