One of the interesting things about limoncello is that it isn't sour at all (if it's made properly). This is because there's no lemon juice in it. The lemon flavor comes from lemon zest--the very outside of the lemon peel, where the essential oils are most concentrated.
In its native Italy, limoncello is most frequently taken cold, as a digestif (an after-dinner drink). I find it especially refreshing early in the evening on a hot day, but it's enjoyable any time you like.
Step 1: Overview
1 750 mL bottle of grain alcohol ( Everclear or similar, also known as rectified spirit--as long as it's potable, strong, and unflavored you'll be fine)
Zest of 8 lemons
Simple, yes? Oh, you'll also need a glass jar in which to keep the stuff. Be sure you have lots of spare room, as you'll add more liquid later. Mine is two liters, and works great.
You want to get the strongest alcohol you can get your hands on. Vodka, even the 100 proof stuff, isn't sufficient. In some states, such as Nevada, you can get 190-proof Everclear, which is 95% ethyl alcohol. Alas, California isn't one of them, so I'll make do with 151 proof (75.5% alcohol, which is still pretty stiff). You can as well, but go with the high-test if you get it. You'll dilute it down to something drinkable later; right now we need a strong but potable nonpolar solvent, and high-proof alcohol fits the bill. I understand an old catalog came with a disclaimer that Everclear was to be used "for the production of homemade cordials," or some such, which is exactly what you're doing here.