Place diced watermelon in a blender container or food processor bowl. Cover and blend or process until smooth - you should have 2 cups watermelon puree. In a 6- to 8-quart kettle or Dutch oven combine the watermelon puree, sugar and lemon juice. Bring the mixture to a full rolling boil (a boil that cannot be stirred down) over high heat, stirring constantly with a long-handled wooden spoon. Stir in the pectin all at once. Return mixture to a full rolling boil; boil hard for 1 minute, stirring constantly. Remove kettle or Dutch oven from heat; skim of foam. Ladle jelly into clean hot half-pint jars, leaving 1/4-inch head space. Wipe jar rims; adjust lids. Allow to cool completely away from drafts, then store in cool, dark place. Makes 8 pints.