Add sugar to a cocktail shaker. Squeeze the juice from the limes into the shaker, then add in the limes.
Using a muddler or back end of a wooden spoon, crush the limes and sugar to release more juices, then give it a quick stir.
Add ice to the shaker and top with the vodka. Close it off with the lid and give it a few shakes.
Pour everything into a cocktail glass and garnish with a lime slice.