I prefer to direct seed watermelon, but you can start them inside in cups as long as you don't disturb the roots when transplanting. Biggest danger is letting them get root bound, they need to be transplanted before true leaves appear. I certainly have misgivings about oak bak mulch, but you will never know until you try. Black plastic is probably a good idea in zone 6. Watermelons have few demands, heat, a loose sandy type soil, and space. Be carefull with hills, 2 inches high and 3 four feet in diameter at most. You don't want to put them up on a conical pile of soil.
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toss the bark. add 20 pounds of composted cow manure per hill. Translating the advice about starting indoors, you can start watermelon seed indoors but the maximum is 2 weeks before you expect to transplant them outdoors. I like to let the cotyledon leaves expand to the point the first true leaf just starts to show.

Watermelons are very sensitive to excess nitrogen. It throws their uptake of potassium and phosphorus into a spin and it causes white streaks and bad flavor in the melons. For this reason, if you use a commercial fertilizer, use something with low nitrogen and high P & K such as 8-24-24.

DarJones