You can either pipe the frosting on (which is how I did it for the photos), or you can make life much easier and use a large (4.25" diameter) ice cream scoop that has a trigger (this one
is the one I have. I used for this technique with my Halloween cupcakes in the below photo). Just scoop your frosting, level it off using a metal spatula (or butter knife), then plop it on top of the cupcake. Then do some minor work after plopping the frosting on there, using the metal spatula or knife to flatten down a bit on top so the frosting reaches out to the edges of the cupcakes, and then to smooth around the sides and on top as needed.