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.

Muddle the lime juice, blueberries and mint leaves. Add the watermelon juice and tequila. Serve chilled in tall ice-filled glasses. Garnish with the mint sprigs