Prep: 1. Juice 1 grapefruit and slice the other for garnish. Make: 1. For each drink, muddle 1 sprig tarragon, 1/4 of the grapefruit juice, and 1 teaspoon sugar in a shaker or mason jar (use a wooden spoon if you don't have a muddler). 2. Add a few ice cubes and 2 ounces of gin (more if you like a stronger drink) put the lid on the shaker/jar and shake thoroughly. 3. Fill your drink glass halfway with ice cubes and then strain in the gin tarragon mixture. 4. Top with 5 ounces of tonic water (or more to taste) and garnish with a sprig of tarragon and a grapefruit slice. 5. Repeat for each drink.