When I make watermelon, I chop the fruit and run it through a press to extract the juice.
I like to use the smaller super sweets (higher sugar) than the large oblong watermelons.
5 basketball size watermelons will give you about 4 gallons of juice and 2/3 gallon pulp.
Watermelon juice spoils quickly, so I like to put my watermelons in the basement for a day or two to pre-cool them before processing for juice.
As soon as you have the juice extracted, add 1 campden tablet per gallon, along with your , yeast energizer, yeast nutrient (optional), tannin, and acid blend.
I measure for alcohol potential (hydrometer) and add sugar to yield a 12-13% alcohol.
Place at 70F or cooler overnight.
After 24 hours add yeast and mix well.
I would also recommend using Cote-De-Blanc yeast, it works well with fruit wines.
After fermentation, racking, and filtering add campden tablet to kill any remaining yeast.
Sweeten back to taste, bottle and let the good times roll.......