Zest lemons with a vegetable peeler, being careful to avoid the white pith. (Reserve lemons for another use.) Put zest into a large glass jar with a tight-fitting lid and add alcohol (there should be enough to cover the zest; if there isn't, add more alcohol.) Set aside in a cool, dark place for 3–4 days. (The higher the proof of the alcohol, the faster the essence of the lemon will be extracted.) When zest turns pale and alcohol has a deep yellow color, strain through a sieve, and store in another glass container. Discard zest.
Combine sugar and 6 cups water in a medium saucepan over medium heat (do not boil). Stir until the sugar dissolves and the syrup is clear, about 10 minutes. Allow to cool.
Pour syrup into lemon-infused alcohol (mixture will turn cloudy) and sample it. Adjust flavor to your palate by either diluting with water or adding more alcohol in small amounts. Then pour liqueur into two clean, dry 750-ml bottles, using a funnel. Close with corks or screw tops. Set aside for a few weeks to allow liqueur to mellow.