This is good to make when you have a giant watermelon that's too big to eat all at once.
A watermelon, a bucket of sugar, a knife, a lime, a bowl, a spoon, and a blender.
Liquify a watermelon as per the instructions for the drink. I suggest you take the seeds out first since they won't just settle out. Also liquify the lime.
Pour the watermelon sludge into the bowl. Add sugar and lime juice to taste. I like to add small pieces of lime too.
Put the concoction in the freezer.
Stir every half hour or so to break up the ice crystals.
but it's a good idea to let it soften a bit before you eat it.