Frost 1 cupcake at a time with a fairly thick layer of buttercream icing. It doesn't have to be fancy but try to keep a nice shape and cover to the edges with the icing.
Place colored sugar in a medium size shallow bowl.
Place a small amount of sprinkles in a small bowl.
Spray the blades of a clean scissors with cooking spray or wipe lightly with a paper towel that's been dipped in oil. Wipe off any excess oil. (If blades of the scissors begin to get sticky at any point, just wash the scissors with hot water, dry well and repeat oil.
Cut about 16-18 marshmallows in half, diagonally and add them to bowl with sugar. Stir to coat sticky, cut side of marshmallow halves with colored sugar.
Cut 1 marshmallow in half crosswise and place sticky side down into sprinkles (or non pareils). Push down on marshmallow half to enlarge it to a circular "button". Remove from sprinkle bowl and set aside. Repeat with other half.
Beginning at the outer edge of the frosted cupcake, place marshmallows all the way around the cupcake. When the outer circle is complete, move in and do the same thing again. The last circle of petals should be right around the "button". (I use 14-16 "petals" around the outer edges. 10-12 around the middle circle and 6-7 around that surrounds "the button.)
With your hands slightly cupped, gently shape the flower so that it's circular and some outer petals aren't sticking out.
Repeat procedure for remaining cupcakes.
The video in the post above demonstrates all these steps.
Enjoy!