• Place the chilled watermelon cubes in a large glass measuring cup or bowl and muddle until broken down.
• Pour the muddled watermelon through a fine-mesh sieve into a pitcher, pressing the solids to extract as much liquid as possible. You should have about 1 ½ cups of watermelon juice. Discard watermelon pulp.
• To the watermelon juice, add the tequila, orange liqueur, lime juice, and simple syrup. Taste and adjust sweetness with additional basil simple syrup to taste.
• Stir well and refrigerate until ready to serve. To serve immediately, add ice to the pitcher and pour into an ice-filled glass for a watermelon margarita on the rocks.
• Garnish with basil leaves.

Optional Sugared Rim:

• Pour basil simple syrup into a shallow saucer, enough to coat the surface. Pour sanding sugar into the second saucer.
• Dip the rim of the glass into the simple syrup. Keeping the glass upside down, let the excess syrup drip off.
• Press the rim into the saucer of sanding sugar, rotating the glass until evenly coated.