Make sure to get all the pulp out. You might need to strain it a couple of times. Once you get all the pulp out, pour the watermelon juice into a large saucepot.
Watermelon is primarily made of water, and it needs to be boiled down more than other jellies. I’ve found that you need to boil for 20-25 minutes to form a proper jelly out of watermelon juice. Once the juice comes to a boil, stirring frequently to reduce boiling.