Watermelon plants should be hand-pollinated if you want to preserve the integrity of the species and avoid cross-pollination with a different type of watermelon plant, or if you maintain a balcony or rooftop garden where insects are few and far between.
Differentiate between the male and female flower. Male watermelon flowers have a stamen that extends upward, beyond the petals, from the base of the flower. At the tip is an anther, home to the pollen. Female watermelon flowers have a pistil, which consists of an ovary at the base of the flower and a long-tube like structure known as a style. At the tip of the style is a stigma, which receives the pollen from the male flower.
Pull back the petals of a fully mature male flower. The stamen needs to be fully extended; it's best to wait until the flower appears to be on the verge of wilting to get a fully ripe, mature specimen.
Brush vigorously against the anther with the artist brush to transfer the pollen to the bristles. Don't worry about damaging the male flower--you're merely helping it accomplish its purpose.
Take the brush filled with pollen and gently brush against the stigma of the female flower. If pollination is successful a big change will occur in the female flower within days. The stem attaching the ovary to the vine will get thicker and the ovary will point downward. The fertilized ovary will gradually transform into a melon.