In a five gallon Jug:
(in the morning)
5 lb sack of sugar (10 cups)
1 or 2 gallons of hot water (to dissolve sugar)
Whole bottle of Root Beer Extract
Let all that sit for a few hours for the hot water to dissolve the sugar and to cool back to room temperature.
Add 6 or 7 lbs of Dry Ice
Fill the rest of the way with water
We always did it in the bathtub because it splatters. Then you can bring it out in a few hours for your party.
One thing to remember: the dry ice can freeze together and block the spigit of the jug. You can try to use smaller pieces, or prop them up in a way that they can't get in the way.
There you have it!