Wash and dry the lemons thoroughly. Using a vegetable peeler, remove only the yellow part of the zest — avoid the white pith, which adds bitterness.
Place the lemon zest in a large glass jar and pour over the alcohol. Seal tightly. Store in a cool, dark place for 10 to 14 days, swirling gently every few days.
Combine the water and sugar in a saucepan. Warm gently over low heat until the sugar dissolves completely. Allow to cool to room temperature.
Strain the infused alcohol through a fine sieve or cheesecloth to remove all zest. Stir in the cooled syrup until fully mixed. Taste and adjust the sweetness as desired.
Pour the limoncello into clean bottles. Let rest for at least 5–7 days to allow flavours to meld. Store in the freezer and serve ice-cold.