Select a planting site that gets full sun, which is at least eight hours daily. Cover the site with 3 inches of aged compost. Till it into the soil at a depth of 8 inches.
Push the tomato cage into the soil at a depth of 6 inches. Firm the soil around the base of the cage so it won't fall over in wind.
Insert two mini watermelon seeds into the soil at the base of the cage, and then insert two more on the opposite side. Space the seeds 4 inches apart and 4 inches from the cage. Water until the soil settles.
Continue watering whenever the soil feels dry to the touch. As the vines reach the cage, secure them to the cage with twist ties. The vines will then grow up the cage on their own.
Stop watering when the fruit has reached its mature size, which will vary with the variety of miniature watermelon. Wait for the fruit to ripen, then pick it by twisting it where the melon joins the stem.