It's actually pretty easy. I zested 10 lemons:
And then, poured a fifth of vodka over the zests, and left them in a sealed plastic container for 2 weeks.
(Giada says to leave them for 5 days, but the traditional Italian way is 40 days. As I ran out of time, and then went out of town, I did 10 days. My Limoncello might have a darker color than Giada's, but I think it also has a better flavor. Or I hope it does.)
Then, strain out the lemon zest (I would seriously put this through cheesecloth or a very fine mess sieve. There's a lot of lemon-peel detritus floating around in there that you probably want out.), add a mixture of simple sugar (3 1/2 c water, 2 1/2 c sugar, bring to a boil, simmer for 5 minutes, then cool), place in a pretty half gallon Ball jar, tie a ribbon around it, and save it for when you want to get good and drunk... in style.
Be sure to freeze this for a while, or at least refrigerate it, before you drink it. Like all vodkas, it's best served ice cold.