On a cutting board, use a star-shaped cutter to cut the watermelon slices. Place them in the freezer until ready to serve. You can do this ahead and keep them in the freezer or simply cube them.
Make the cocktail:
Pour the tequila, Cointreau or triple sec, and limeade into a pitcher. Stir to combine and refrigerate until ready to serve.
Serve:
Add 2 tablespoons of blueberries to the bottom of each glass. Fill with ice, ¾ of the way up, top with cut watermelon.
Pour the mixed cocktail into the prepared glasses. Add a slice of lime, for garnish, and enjoy.
Notes
Make it a pitcher, 8 servings, by doubling this recipe and combining 1 ½ cups tequila, 1 cup Cointreau or triple sec, and 4 cups limeade into a pitcher. Refrigerate it until ready to serve.