Here's how it works:
Step 1: Prep the cake. If you're using two rounds, level the cake and fill it. Whatever you're using, do a base coat of frosting. This will be your background color. (In retrospect, I could have chosen something with higher contrast.
Step 2: Choose your coloring page. I did an internet search of printable coloring pages. Then I laid out parchment paper and traced the bottom of the cake pan on it. This will give you a "window" to try to fit the page in. I realized we wouldn't have room for the balloons in the original coloring page, so I knew to leave them out.
For a professional touch, add a border to the top and bottom edge of your cake.