How to get sprinkles to stick to the side of a cake?
This is a little hard to do without making an epic mess.

First, lay out a big piece of parchment or wax paper to catch the sprinkles that you are about to throw everywhere.

Place your cake on it's platter over the parchment paper and tilt it so that you can apply sprinkles to the uppermost side. Don't tilt it so far that it slides off! Apply the sprinkles with your free hand, pressing them gently into the frosting (some suggest wearing a glove, I don't). Turn, and repeat. Periodically collect all your sprinkles from the parchment paper.