Rather like Genepi, making Limoncello is pretty easy. Its all about the ingedients.
YOU NEED UNWAXED LEMONS FROM THE AMALFI COAST IN ITALY. Accept no substitutes. Seriously using standard lemons will make you crap limoncello.
You also need high strength alcohol. Ive seen recipes using 40% vodka as their base alcohol. If you want to make Limoncello like the Italians you need 96% alcohol. 96% isnt available in France but I bought 96% from Italy and Poland.
From personal experience I would recommend buying a decent (sharp) peeler and peel at least 9 large lemons.
Place the peel in a mason jar and add 1 litre of alcohol.
Seal and leave in cool dark place.
After 4 days the lemon peel has given up all its goodness but I left mine for 24 days after the recommendation from an Italian nonna.
When filtering the liquid I recommend first passing it through some muslin to catch the big stuff and then passing it through a neutral coffee filter.
If you want to know how to make simple syrup you can either boil water and sugar until it dissolves or shake cold water and sugar together until all the sugar has dissolved.
I measured and marked a 33cl level on the 1 liter bottle and filled to that level I then diluted with a simple syrup bringing the alcohol content from 96% to a more palatable 32%.