With a veg peeler, peel off the zest of 15 lemons. (I juice them as well and also add that usually, although might be making lemonade syrup this time).
I can't remember the exact quantity of sugar, I just lob in about 3 cups and adjust at the halfway mark if necessary.
Put the zest, juice, sugar and a 70 cl bottle of vodka ( cheap supermarket vodka is perfect) into a large container, close it, give a good shake and put in a cool, dark place. Shake daily for a week to make sure sugar dissolves, then weekly.
After 40 days, the original recipe called for the sugar to be melted into boiling water (70cl), cooled and added, but as I use the juice, I add it at the start. But I will check the flavor at 40 days, add more sugar if needed and possibly some water. Shake weeekly for another 40 days.
Strain out the zest, bottle into smaller bottles, put the first into the fridge (or freezer) to get well chilled to do a proper quality control test. I tend to serve it diluted with 7up or San Pelegrino Limonata as a lovely pre-dinner grown up lemonade, or occasionally as an actual shot after a meal.