Spread a small amount of frosting on each cupcake. It doesn't need to be perfect, since most of it will be covered by toppings.
Place a gumdrop or round candy piece in the center of each cupcake.
Flatten each of your marshmallows with the palm of your hand, then use a knife or round pizza cutter to slice each marshmallow into 4 even slices.
Dip the cut side (or the side which curls inward, if both sides are cut) into the colored sugar or sprinkles. Place each of the slices, sugar or sprinkle side up, onto the cupcake around the center gumdrop, in a flower petal pattern.