Separate the mini marshmallows by color. Cut the marshmallows in half diagonally to create petals. You will need about 15 marshmallows for each cupcake.
If using store bought cupcakes, remove some of the frosting from one cupcake and smooth the frosting so that it's flat across the top. If using homemade cupcakes, frost one cupcake, smoothing the frosting so it's flat across the top.
Place marshmallow halves around the outer edge of the cupcake in a circle, making petals as shown. It can be helpful to use a toothpick to press the marshmallows down into the frosting so they stick. I used approximately 15 petals on the outer circle.
Create another circle just inside the outer one, using about 10 marshmallow halves as petals. Then create one more inner circle, using 5 more marshmallow halves as petals, and an additional petal in the very center if desired. Gently press all the petals down into the frosting to make sure they stick.
Repeat with remaining cupcakes. Store in a covered plastic container if possible.