1½ oz. fresh lime juice--not from jars, plastic squeeze tubes nor Hi-C
1½ oz simple syrup [q.v. right here on Instructables]
4 oz. white rum...I like Cruzan but your fave is best
Shake with ice cubes in a cocktail shaker. [You must smile your biggest Cuban smile while shaking.] Shake it a long time! If you keep your rum and glasses in the freezer, and if your ice is cold enough, you may see tiny crystals in the drink when you strain it into two martini glasses--that's ideal! You could garnish with a lime wedge or twist, but I like a Maraschino cherry because that unique color is so great against the pale, pale green. For the second round, you won't have to force the smile...