Shake vigorously for about 15 seconds, until the shaker is icy cold.
Fine-strain into a chilled martini or coupe glass.
Garnish with a small watermelon wedge or a lemon twist.
Notes
Use fresh, strained watermelon juice for the cleanest flavor. If your watermelon is very sweet, reduce the simple syrup. For a lighter spritz, top with a splash of sparkling water. Chill spirits and glassware ahead for best texture and temperature.