Combine the cubed watermelon and sugar in a bowl or jar and stir well
Refrigerate for overnight, stirring once or twice, until very juicy.
Add the vinegar and stir again.
Refrigerate another 12 to 24hours.
Strain through mesh strainer into mason jar.
Add 1 to 2 tbsp of shrub and mix with 8 oz. of sparkling water to serve.