Wash lemons then zest the lemons and set aside. Filter the vodka (unless you bought already filtered). You can use a water filter for this. Combine zest and liquor into one gallon glass jar, screw lid on tight.
Second Stage- Adding Sugar
45 days later, shake the zest/vodka mixture up. Make simple syrup by boiling the sugar with the water until the sugar is dissolved. Add half of the simple syrup then taste and add additional syrup to taste. (I ended up only adding half of the simple syrup and it was sweet enough for me). You can always add more sweetness as you make cocktails with the limoncello later.
Final Stage - Filtering
45 days later, filter the liquor through a screen mesh or permanent coffee filter, then through an unbleached paper coffee filter. Sterilize the bottles you plan on storing the liquor in. Pour the lemoncello into the prepared bottles using a funnel to prevent a mess. Finally, pour yourself a drink or hop in the car to deliver some lemoncello to your closest friends.