Cultivate

I once asked my father about this as he owns a gardening and landscaping company. He said he thought it would be too difficult for an individual to do as it probably involved cross breeding varieties of watermelon to a very specific variant and then making sure it kept producing fruit or that you kept taking cuttings and regrowing these.

I asked my father about this once as I love seedless watermelon, and he said that he thought it would be difficult for an individual to do on their own and that he wasn't entirely sure of the process, but that it probably involved a lot of time and selective breeding of a strain.