Chop watermelon into small cubes and freeze for 2-4 hours in a single layer to avoid clumping. Once the watermelon is frozen, you can then transfer it to freezer bags until ready to use, or go right to step 2. 2. Add frozen watermelon, sweetened condensed milk, lime juice, chopped basil and basil simple syrup to a blender. Blend until smooth, and transfer to a loaf pan or airtight container. Wrap tightly with plastic wrap or place lid on, and freeze for at least 4 hours, or overnight. Scoop and serve as desired! *Basil Simple syrup (makes about 1 cup):Bring sugar, water and basil leaves to a boil in a small saucepan. Once boiling, reduce heat to medium and simmer, stirring occasionally for 5 minutes. Turn off the heat and allow to cool completely. Strain cooled syrup into a jar or airtight container and refrigerate. Lasts 2 weeks.