Put the blackberries in a cup and stir until the berries break apart. Mix around 5 teaspoons of simple syrup with the mashed berries. If you prefer a sweeter drink, add 1-2 additional teaspoons of syrup to your drink.
Next, add the gin and fill the glass with ice. Drink a little with around 3 ounces of water, and mix everything gently with a spoon. You may decorate your glass with some mint leaves or a lavender sprig!