1 part lime syrup (e.g. Rose's Lime Juice)
2 parts vodka
a lime

ice cubes

cocktail glass

Put several ice cubes, the vodka and the lime syrup in a glass and stir. Strain (ice cubes remain in the glass used for stiring) into a cocktail glass and garnish with a slice of lime.

Don't confound fresh lime juice with lime juice syrup. Lime juice is pure fruit juice, while lime juice syrup is made of lime juice, water and sugar. The problem for all the confusion: a famous lime juice syrup is called "Rose's Lime Juice" although it definately is a syrup!