Add your basil leaves and the juice of 1 lime to a cocktail shaker. Muddle the basil leaves for 1-2 minutes.
Add the watermelon to the cocktail shaker and muddle that down until it's mostly liquid.
Add 1½ ounces (1 shot) of gin to the cocktail shaker.
Add some ice to the cocktail shaker and shake thoroughly for 1 minute.
Strain into a glass filled with ice.
Optional: Garnish with more basil and watermelon.