I used a cupcake wrapper template over at Skip to my Lou estimated a circle for the bottom and top of the cupcake once I sewed the wrapper template up.
Then I used this tutorial over at Sew-mama-Sew for the top of the cupcakes for the frosting. You can add beads for sprinkles and a cherry if you want too. After I made the cupcake I decided to add some pleating to the outside to look like the traditional cupcake wrapper. Just pinch vertical lines and straight stitch down them from top to bottom to create the pleating.