From http://www.foodandwine.com/recipes/watermelon-tequila-cocktails

2 oz silver tequila
1/2 oz fresh lime juice
1/4 oz simple syrup

2 cups diced seedless watermelon (1 pound)
~5 blueberries
1/4 cup loosely packed chopped fresh mint leaves, plus some garnish

In a blender, puree the watermelon until smooth. Set a fine-mesh strainer over a bowl and strain the watermelon juice, pressing gently on the solids to extract as much juice as possible. Discard the pulp.