Tutorial: How to Decorate a Sprinkle Cake

A few weeks ago I made a rainbow sprinkle cake as the Kid's Cake at a birthday party that featured a bigger main cake that wasn't gluten free. Since posting photos of the cake, I've received several requests for instructions on how to create one.
 It was actually one of the easiest cake I've ever decorated, since it's almost impossible to mess up. I think a round cake works better than a square or rectangular one, and would be easier to decorate.

Let's get started!