The basic idea is pretty simple, but very important if you don’t have a recipe to go off of. Just substitute half of the water in the directions with the desired liquor. Dissolve the jello with the hot water and add the alcohol to it only after the water/gelatin mix is cool to the touch. If you add the alcohol while the mixture is still hot, it will evaporate. You do NOT want your alcohol to evaporate. Can you say “Bye, bye party!”?
The majority of recipes on this site are proportioned to one 3-ounce box of gelatin. Each box should make about 20 gelatin shots, depending on the size of the cups you use. While 20 sounds like a lot, it will probably only serve 3 to 4 party animals. If you’re planning a huge party, you may want to stock up on some more jello.