Cut a small watermelon in half and quarter a small watermelon. Scoop the watermelon flesh from the rind with an ice cream scooper and put it in a large glass measuring bowl or a large bowl until you have 4 cups of watermelon flesh and watermelon juice.
Add the freshly squeezed lemon juice into the bowl with the watermelon pieces. Sprinkle the sugar into the bowl and mix until the sugar has dissolved.
Pour the melon mixture into a blender or food processor. Pulse to break the watermelon pieces up well and then hit the puree button process until all the pieces of watermelon are liquified.
Pour the sweetened watermelon puree into a glass 9 x 13 baking pan. Cover with plastic wrap.
Place the pan in the freezer.
After one hour take the pan out of the freezer and break up the ice crystals with a fork. Run your fork around the inside edges of the pan also to incorporate those frozen bits in with the rest of the mixture.
Repeat this process every hour until the granita is completely frozen and broken up into ice crystals.