Place the mint leaves in the bottom of a cocktail shaker, pour in the lemon juice and muddle gently using a muddler or the back of a spoon. Be careful not to over-muddle, as this can make the mint bitter.

Add the gin and simple syrup to the shaker with the muddled mint and lemon juice. Fill the shaker with ice, secure the lid, and shake vigorously for about 15 seconds.