Watermelon fruit is a tricky crop to grow in the North of the UK due to the high temperatures and long growing season required to ripen the fruit. Text books quote the temperature requirements as a daytime temperature of 30 C and a nighttime temperature of 24 C, which is beyond the capabilities of the typical cool greenhouse. Therefore to have any chance of success it would have to be grown in a Greenhouse, after choosing one of the quicker maturing varieties to grow. It is possible to succeed doing it this way, but consider whether it is worth the effort and space used in the greenhouse.
Sow the seeds singly in a small pot compost at about 20 C. Grow on until you are ready to transplant into grow bags in a Greenhouse. Train them as single cordons with the main stem trained up a cane or wire to the roof of the house. Allow the side shoots to develop as they carry the fruit. Pinch out the side shoots at the second leaf beyond the fruits. Hand pollinate by picking the fully open male flower, fold back their petals and place over the female flower to transfer the pollen. The female flower has a fruitlet at their base.
Feed with liquid tomato fertilizer until the fruit start to ripen when you will notice a change of the skin colour. Once all the fruit on your plant are ripening, water should be gradually withheld to hasten the ripening process. Only allow a maximum of 4 fruits to grow per plant.
As the fruits are so heavy, you will have to support individual fruits with a lattice bag, such as an old onion bag, placed around them and supported from the roof. This takes the weight off the stem of the plant, otherwise the plant stem would buckle and maybe break.
Pests and diseases. It is the usual suspects of aphids, white fly and red spider mite, which should be treated in your chosen method. Powdery mildew may also be a problem.
Blacktail Mountain. Claimed to produce a reliable crop of 25 cm dark green fruit, which will produce scarlet flesh provided it gets extra heat. Allow 75 days in the South of the UK, and 90 days in the North of the UK from transplanting to harvest. Claimed to be able to withstand temperatures as low as 7 C at night.