Cut your watermelon into the desired shape.

I began with a  small round seedless watermelon and cut the watermelon into a rectangle. For obvious reasons, a seedless watermelon does promote friendly consumption.

Frost with your choice of store purchased or homemade whipped cream. I have tried this cake with the canned whipped cream and homemade whipped cream or coconut frosting.

The taste of the homemade whipped cream was superior; however, for ease and consistency, the canned whipped cream was easier to apply.

After choosing the frosting (a coconut milk based whipped cream is also a good choice) frost the watermelon cake just as any other cake.

If you are making your own whipped cream please remember a  thicker consistency is desired as it adheres better to the watermelon and allows for easier decoration.

Feel free to use the whipped cream frosting of your choice. There is a little trial and error involved to perfect the consistency which will adhere to the cake.

It will be apparent very quickly if your frosting needs to be thickened a smidge to adhere to the watermelon cake.

There are quite a few recipes for coconut frosting listed at Allrecipes if you are interested in trying a coconut frosting. I find the reviews invaluable for tips and tweaking of a recipe.