Very thoroughly wash and dry the outsides of the lemons. With a vegetable peeler, peel the lemons, avoiding the white piths, which are bitter. Place in a 32 ounce jar.
Pour the vodka over the peels. Tightly seal and store in a cool, dark place for at least 4 days or up to 1 month (1 week does the trick for me).
Strain the infused vodka: Place a fine mesh strainer over a large bowl or a 4-cup or larger liquid measuring cup with a spout. Carefully pour the vodka/lemons though the strainer. Discard the lemon peels.
Make a simple syrup: Combine the sugar and water in a small saucepan. Bring to a simmer and stir to dissolve the sugar completely. Let come to room temperature (this will help the final limoncello to stay clear; you can speed the process along in the fridge).
Pour 1 cup of the simple syrup into the infused vodka to start. Stir to combine, then taste and assess. If you’d like it sweeter, add more of the simple syrup until you are happy with it. (NOTE: the next time around, if you’d like to keep the final liqueur even boozier but just as sweet, you can change the ratio of the simple syrup to 1 ½ cups sugar to 1 cup water.)
Transfer your limoncello to your desired storage containers (I use 2 16-ounce swing top bottles). Refrigerate or freeze for at least 4 hours prior to serving. Store in the refrigerator for up to 1 month or freezer for 1 year.
Notes
TO STORE: Once bottled, homemade limoncello can be refrigerated for up to a month or frozen for up to a year. When frozen, it takes on a slushy-like consistency because the high alcohol content prevents it from freezing through.