You'll need:
2 oz gin
1 1/2 oz sweetened lime juice, such as Rose's
2-3 rounds of lime
Sugar, for the rim
Round of lime for a garnish
Squeeze some lime juice around the rim of a martini glass. Place some sugar in a saucer, and swirl the sugar around the rim to coat, and place the glass in the freezer to chill. Over ice in a cocktail shaker, layer the gin, sweetened lime juice, and the juice from the rounds of lime into the shaker as well as the squeezed rounds. Shake to combine. Serve in the chilled glass, with an extra round of lime for a garnish. Enjoy!