Wash the lemons by scrubbing them with the rough side of a sponge. Dry them with a paper towel or tea towel.
Peel the lemons with a vegetable peeler, taking extra care to not leave too much pith (the white part of the lemon skin) on the peel. Too much pith will make the limoncello bitter.
Add the lemon peels to a large glass jar with a lid. Add the grain alcohol to the jar.
Store the jar in a cool dark place out of direct light (like a closet). Let the alcohol infuse for 30 days. Every few days, shake the mixture up a bit so the lemon peels move around.
Once the 30 days are over, prepare the simple syrup. Place the water in a pot and heat it over low heat. Once the water is slightly warm, add in the sugar, stir and let it dissolve completely. Once the sugar is dissolved, let the mixture cool completely to room temperature (a few hours).
Once the simple syrup has cooled, prepare a sieve with a coffee filter or cheese cloth inside. Place it over a large bowl and strain the alcohol-lemon peel mixture into the bowl, so that only the liquid collects in the bowl.
Add the simple syrup to the infused alcohol (it will go slightly cloudy) and stir with a wooden spoon. You now have limoncello!
With the help of a funnel, pour the limoncello into bottles.
Let the limoncello rest in a cool dry place out of direct light for 30 days. Letting the limoncello rest will allow the flavors to meld together and become smoother.