Cut watermelon in half lengthwise 1-inch above the center. Remove pink fruit with melon ball cutter or half teaspoon measuring spoon. Remove seeds. Scrape out remaining fruit and drain well.

Measure around rim to decide how wide to make scallops. If it measures 45 inches there is room for 15 (3 inch) scallops or 30 (1 1/2 inch) scallops. Cut a cardboard pattern the desired width of the scallops at the bottom edge.

Trace pattern with knife tip around outside edge of melon. Cut with small sharp knife. Chill melon until ready to fill with melon balls.