Carefully remove the zest from the lemons using a microplane or grater, being careful not to remove any of the white pith, which will make the limoncello very sour. Reserve the lemons to squeeze and freeze the juice for later use.
Put the lemon zest into a mason jar or other similar glass container of at least 2-quarts in size. Pour the vodka over the zest, and let sit in a cool dark place for 7 days.
After 7 days, strain the liquid through a fine mesh strainer lined with muslin, a coffee filter paper towel, or several layers of cheesecloth and return the vodka to the container. Discard the lemon zest and pour the vodka mix back into the mas jar or a spring cap bottle.
Combine the water and sugar in a small saucepan set over high heat and cook, stirring continually, until the sugar is completely dissolved, approximately 3 minutes. Once completely cool, add the sugar syrup to the vodka, and stir to combine. If you like it sweeter, make and add a little more syrup based on the 1:1 ratio of sugar to water.
Cover and put the container in the freezer for at least 4 hours. Limoncello can be stored in the freezer for up to a year.
Nutritional information is provided as a courtesy and is sourced from the USDA Food Database.