Wash the lemons with a vegetable brush or plastic scrubber under very hot water; pat the lemons dry. Zest the lemons with a microplane zester or vegetable peeler so there is no white pith on the peel. You may need to trim away any large pieces of pith with a paring knife.
Transfer the lemon peels to a 1-quart jar and cover with vodka. Screw on the lid.
Let the vodka and lemon peels infuse in a cool, dark place for two weeks or as long as 45 days. (The longer you allow it to rest, the better it will taste.)
After macerating for two weeks (or longer), the alcohol is ready to be drained and mixed with the water and sugar.
Line a strainer with a large, flat-bottom disposable coffee filter and set it over a 4-cup measuring cup. Strain the infused vodka through the filter.
In a large saucepan, combine the sugar and water; cook until thickened, approximately 5 to 7 minutes. Let the simple syrup cool for one hour before adding it to the infused vodka. Stir gently to mix.
Insert the funnel in the neck of one of the bottles and fill with limoncello. Repeat with remaining bottles.
Chill the limoncello in the refrigerator or freezer for at least 4 hours before drinking.