You're making jigglers, here, but substituting half the water for alcohol. Mix the jello in until its well disolved, pour into a container and place into the refrigerator. Tinfoil pans and trays work pretty well also. Be careful in the type and volume of alcohol you use. We've found that adding too much will tend to cause the sugar in the jello to precipitate out and leave the bottom gritty.
Rum hides better within the jello than vodka, though either works pretty well.
Refrigerate it for about 45 minutes or until it begins to take on a jelly-like consistency. It should stick to your finger a bit when you touch it. Just remember that you're fiddling with the hydration a bit, so adjust accordingly. This is Roy's oft-requested drink, a nice summertime concoction. Whereby once it is tasted, the drinker cannot seem to get enough.