Find a location to plant the watermelons that is in full sun so the fruit gets at least 7 hours of sun per day. It should get good air circulation, but if possible, be protected from high winds, such as on a south-facing hill.
Purchase watermelon plants a local nursery or gardening center, planning on transplanting them when the temperature is around 65 degrees Fahrenheit.
Dig a hole with a shovel that is about 2 feet in diameter and 1 foot deep. Combine sandy loam, compost, and well-cured manure, equal parts. Make sure the soil has good drainage, usually provided by the sandy loam or a sloping hill planting site. Watermelons will grow best in neutral soil.
Plant the transplants carefully into the ground at the depth they were growing in their pots, setting the roots into the hole straight down. Space the watermelon plants around 3 feet apart for smaller varieties, and 12 feet apart for larger varieties.
Layer a thick mulch to hold in moisture and prevent weeds. It will also keep the melons clean as they grow.
Water the watermelon plants with about 1 inch to 2 inches of water per week, depending on how dry your climate is.
Cover the plants with floating row covers to keep the air warm. Take off the covers once the flowers bloom on the watermelon plants in order for them to be pollinated.
Prune the first small shoots with pruning scissors (for smaller shoots) or pruning shears (for larger growth) from a watermelon plant in order to encourage stronger, healthier growth. This is important when the plant is young, but once it is more mature, it is not necessary to prune the watermelon plants often.
Harvest the watermelons about a month after they have bloomed; this is when they should be fully mature.
Keep the watermelons protected from cold weather if necessary. Use space heaters within 5 feet of the watermelons for severe weather, or else just cover the watermelon plants with blankets, pinned down on either side with rocks as weights to keep heat locked in.