Either put a sticker on your body. And get tanned or burnt, draw a shape you want with sunscreen, like a heart and fill it in, or put sunscreen everywhere but a certain spot and do an outline of the shape you want (tho that seems a bit harder)
Draw a design on paper cut it out place it on you, with a little tape underneath to hold it. Get out on the ground in the sun & lay there for a while. Check it if not darkenough stay there till the design stands out.