For a joint party we had a Luau my 15 year old wanted a zebra Flip Flop Birthday Cake. We baked a regular box mix in an 11 by 13 pan. I made a pattern from regular printer paper and cut it out.
I froze the cake for about 10 minutes after cooling. Then I did a thin layer of icing and refroze for about 5 minutes. I took a plastic knife and a bowl of water. I dipped the knife in the water and used it to smooth the icing. I refroze for about 5 minutes.
Then I took a regular kitchen knife and traced the flip flop pattern on to the cake. I had already mixed the color up so I smoothed it on with a small spatula following my pattern. I froze for 5 minutes and smoothed the icing out with the plastic knife and water again. I froze for about 5 more minutes.
Then I drew lines out of black colored icing. I piled green icing to make the straps and covered it with green sprinkles. We refroze the cake until the morning of the party. We did it 2 days ahead.