Use ripe, yellow lemons, rinsing them well before use.
Using a course zester or vegetable peeler, remove just the yellow skin of the lemons. Avoid the white flesh under the peel. Place the lemon peel in a large glass jar and pour in the vodka.
Seal the jar with a lid and let infuse for one week at room temperature. The mixture will turn a beautiful yellow color.
After one week, combine sugar and water and bring to a boil. Boil for 15 minutes without stirring. Allow the syrup to cool to room temperature.
Pour the infused vodka mixture through a strainer into the syrup. If you want pulp-free limoncello, pour the vodka mixture through a coffee filter.
Pour the filtered mixture into glass bottles and seal. You can store the limoncello at room temperature, where it just gets better with age. Before using the liqueur, place the bottle in the freezer and serve ice-cold.