All you need is
Nail polish and sunscreen
Step 1: Apply
Take you nail polish and apply it on your skin. Then take the brush and make the design of your choice.
Step 2: Swim
Play and run on on beach. Or swim in the ocean or the pool. Or just lay out and tan.
Step 3: Take It Off
After your long day in, get your nail polish remover and take off the nail polish. And look, the sun hastened your skin and left a cool design on the area where your design was placed.
I wasn't in the sun that much. But, I could see a big difference.