First of all, roll some brown fimo in a ball and then push it down on to a surface so that the bottom half flattens out.
Take a big lump of white fimo and roll it flat.
When the fimo is really thin, flat and long, use a knife to cut the top and bottom edges straight. Lightly use the knife to create little ridges all along the white strip - this it to make it look like a cupcake case.
Wrap the white strip around the brown ball.
Place some pink fimo on top in the style of icing and push a screw eye in to the top.
Roll a small lump of white fimo until it becomes a really thin strip. Use the knife to cut it in to small rectangles, like sprinkles.
Place the fimo sprinkles on top of the cupcake and bake in an oven until hard.