Shave off the yellow parts of the lemon rinds using a potato peeler, and then put them into a large glass bottle with the alcohol. Cover the bottle with plastic wrap to prevent evaporation and let rest 12 days at room temperature. Bring 1 1/2 liters water to a boil, and then remove from the heat and stir in the sugar until completely dissolved. Let cool. Strain out the alcohol, discarding the rinds, and stir into the sugared water. Pour into a sealed glass bottle and let sit an additional 10 days. After 10 days, the limoncello is ready. It is best served chilled as a digestive after a meal.