We shade space between them, thus sometimes leaving small white sites. After that slightly we put a shadow from below, from above, on the right and at the left, the farther from the center, the more darkly. Part of the middle of a water-melon it is left not touched, light falls there. The water-melon is ready. If you wish that it was very realistic, find a water-melon in the Internet and thoroughly draw in detail a pattern on a water-melon (i.e. a dark coloring).
You can watch video on realistic drawing of a water-melon below.