Chop the flesh of the watermelon roughly and, in batches, puree in the food processor for a few minutes. As each batch is pureed pour it through a fine mesh sieve into a pitcher. If you want to make sure juice is totally pulp free strain the juice once more. Taste it, too, and if you think it could use a little sugar boost add a spoonful or so of sugar but not much more than that—the point isn’t to make a simple syrup out of it.