Propagation
Sow fresh seeds in situ in pairs 25 mm (1 in) deep in a prepared raise garden bed, then thin out after germination.
  
Watermelon seeds germinate at temperatures of  70 to 90°F and take 4-6 days to emerge then should be hardened off for one week  then planted out in 4-6 weeks in a well-drained prepared bed with a soil temperature between 20º to 30º C. (68º to 86º F).

The best months for planting out watermelons in Australia are from April to July and the fruit can be harvested in 9-14 weeks. In the United States in Zone 9 the best month for planting is February March or during August.

Hint:
To determine when a watermelon is ripe there are four things that you can look for,
1) The associated tendril turns brown.
2) Where the fruit joins the stalk a brown halo starts to appear.
3) Checked the base of the fruit where it touches the ground and see if it is turned from yellow to white.
4) Yellow patches on the top of the fruit should not be mistaken for disease as it is properly sun burn, just cover the fruit with a piece of cloth.

Botanic information
Leaf: The dark green variable palmately lobed leaves are up to 400 mm (16 in) long with 5-7 lobes that may be dissected or lobed.  The pubescent petiole is up to 250 mm (9¾ in) long and the leaf upper surface has silky hairs.

Flower: The plant is monoecious with male and female flowers. The bright yellow broad campanulate flowers are up to 60 mm (2⅜ in) or more wide with 5-obovate deeply veined petals. The male flowers have 3-seperate stamens and the female flowers have an inferior ovary. They appear solitary in the leaf axils during summer.

Fruit: The dark green variable berry is oval to oblong up to 500 mm (20 in) long and may be striped and weigh up to 14 kg. It has sweet and juicy red or yellow flesh with white or black seeds. The fruit matures typically between 85 and 95 days.