Here is the process that can be used to make this. True Limoncello is
made with high alcohol spirits and very specific lemons in Italy but
this turned out well. I made it with differing qualities and proofs of
vodka and did not consider the differences substantial to the final
taste.

Limoncello recipe
Rev 1

Starting point:

750 ml 100 proof vodka: Jacquins or any inexpensive brand is
acceptable (100 proof is preferred, 80 proof will make just a lighter
alcohol Limoncello.)
375 grams (1 2/3's cup) table sugar (cane or beet, it does not
matter)
7 medium lemons, good color.

Procedu
Zest lemons, if you are removing more than 2.5 g per medium lemon you
are probably taking off too much pith with the zest. You should end up
with about 18 g +/- 10%. A fine cheese shredder makes a good zesting
tool. The goal is to take off the outer surface of the lemon; if the
remaining 'pith' (outer covering) is white, too much has been
removed; a pale yellow pith should remain. Pith will impart a bitter
taste to the end product. The lemons can be juiced and the juice
frozen for another purpose; it is not used in Limoncello.

Add the zest to the vodka and let it steep several days at room
temperature (at least 65F); a week is fine. Once you see good color
you have probably extracted enough oil from the zest. The zest will
start to lose its color; it will not get to white but will become a
pale yellow. If you want to experiment with steeping times pull off a
known quantity of liquid, add sugar (1/2 the amount of liquid) shake
until dissolved and taste. (I.E. 50 ml liquid to 25 grams of sugar.)

Remove the zest from the liquid; pour it through a coffee filter if
preferred.

Optional: Fine with hot mix Sparkolloid at a rate of 1 g/gallon (US)
and rack when clear. This will give a clear end product. This step
requires the liquid be at room temperature, the fining agent is not
effective at cool or cold temperatures. Commercial Limoncello is often
cloudy; this step is only for aesthetics.

Add the sugar and stir or shake until dissolved. It will not look like
it is possible to dissolve this much sugar, but it is possible. Just
shake it up, let it settle repeat until all of it is dissolved.

Serve chilled preferably but the amount of sugar added makes this
smooth tasting at any temperature.

The amount of zest used, amount of sugar added and steeping times are
variable by personal preference; the only hard and fast rule is the
strength of the vodka and that is not necessarily hard and fast either.
I have made it both ways and liked the end result of both.

Joe