Shake vigorously for about 15 seconds until very cold.
Fine-strain into the prepared glass and garnish with a maraschino cherry or lemon twist.
Notes
Balance to taste: add a splash more simple syrup if too tart, or extra lemon if too sweet. For a lighter version, top with a splash of soda water. Chill the vodka and glassware for an extra crisp finish; fresh juice makes the flavors brighter.