First things first: clean those lemons! You don't want any wax or pesticides in your limoncello. I buy organic lemons if I can find them for a reasonable price and use a mushroom brush to give them a light scrub underwater, then a quick rinse.
Use a vegetable peeler to remove the yellow rind from all of your lemons. Avoid the white part (aka the pith) as this will make your final product bitter.
Throw the peels in a large container with a lid, pour the alcohol over the peels and put the lid on it. Give it a good shake, because, well, shaking things is fun and gets those oils from the lemon peels moving, place it somewhere cool and dark for at least three weeks.
After three weeks have passed, open the jar and strain the mixture, discarding the lemon peels. ( See Notes )
Now, time to make the simple syrup which will sweeten your limoncello! I like a light syrup, so. Bring 6 cups of water to a boil in a large pot. Turn the heat off and stir in the sugar, stirring until the sugar is completely dissolved. Allow it to cool.
Once cooled, add the lemon concoction to the simple syrup, stir to combine and pour into two 750-ml glass containers. It's best if stored in the freezer.