what actually works to cultivate watermelon?

They want heat, sun and room. Don't plant out until the soil is properly warm, cold soil just stalls them for weeks. Full sun, rich soil with plenty of compost, and space, the vines run a long way. Water deeply while they're growing and flowering, then back off a bit as the fruit ripens, it gives sweeter melons. Short summer where you are? Pick a small quick variety. To tell when it's ready, the curly tendril nearest the fruit goes brown and dry, and the patch underneath turns creamy yellow.