what is the best way to savw seed??????
watermelon, cucumber and squash.
All of these will cross readily, so if you are growing different varieties of these within the same species, you're going to have to isolate them (usually half a mile), or hand-pollinate them.
For saving veggie seeds, the book "Seed to Seed" by Suzanne Ashworth, is worth 50 times its weight in gold. Definitely check it out and read up on the Cucurbitacea family.