Plant between 12-24 inches apart. They also like rich soil and lots of water. Watermelons like consistently warm conditions so plastic mulch and row covers can assist.
Harvesting watermelon at proper ripeness is an art. Thumping should produce a low, hollow sound. Spread thumb and forefinger and press hard on fruit. If you feel any give, watermelon is ripe. You may also look for the tendril nearest the point on the vine where the fruit stem attaches is brown/dead or the spot where the fruit rests on the ground is yellow.