1/2 (8 cups) gallon milk
1/2 cup - 1 cup powdered milk
4 oz yogurt with live cultures
your choice of flavorings and sweeteners
Put 8 cups of milk (2% or whole, I've never tried lower fat but I'm sure it would work) in a crock pot on low for 2 1/2 hours. Depending on how thick you want the yogurt to be you can add powdered milk (1/2 cup for medium consistency, 1 cup for thicker consistency) or for a more jell-o like consistency use 2 pkts unflavored gelatin (although if you used flavored I'm thinking it would make flavored yogurt...but I've never tried that).
After 2 1/2 hrs unplug the crockpot and leave it be for 3 hrs.
Add 1 cup of the warm milk to 1/2 cup yogurt with live cultures (any brand works as long as it has live cultures. I like Dannon all natural or Oinkos). Once again you want to use plain instead of flavored unless you want to flavor the entire batch. The stir the yogurt-milk mixture back into the milk in the crock pot, leaving the crock pot off and wrap the whole thing in a bath towel. Leave it to culture overnight.
In the morning put it in the fridge. I like to add 3 TBLS of agave or 1/4 cup sugar and 1 1/2 tsp vanilla to mine and then divide it into 1/2 cup tupperwares that my kids can just grab out of the fridge. You can also add fruit puree to flavor it, or fresh fruit as you serve it.
Good luck! Happy yogurt-ing!