It's been done before, and blogged about before. But I wanted to record my sister's method here for others to reference with ease. Before I launch into the recipe, allow me to describe this yogurt's virtues.
First, it is cheap. I used to spend $3.79 (or sometimes, if I was lucky - $3.00) on a 2 pint container of organic, full-fat yogurt. Now I can spend $1.89 (or $3.19-3.69 if organic) on a half-gallon of milk (plus say $0.50 max of store-bought yogurt "starter") and turn it into twice as much yogurt. Second, it is tasty. I would liken the taste to Straus, which is now sadly unavailable to us in KY. It is tangy, but not overly so, and it is thick, but still smooth. Finally, it is versatile. You can strain it if you want Greek yogurt.
Yogurt a la The Cheesy Sisters
1. Obtain a crock-pot with a temperature probe function. I use this one.
2. Get a half-gallon of whole milk. (I have had the best luck with milk that is not ultra-pasteurized. Unfortunately, around here, this means the milk that I can find which fits these specifications is not organic. Try a couple of different types and see what works best for you. I strongly suggest using whole milk. You will not get the same texture or taste if you use a less-fatty - inferior - milk.)
3. Get about 2 heaping tablespoons of plain yogurt. I use Stonyfield plain yogurt. I have used full-fat and low fat with no discernable difference in results. (As I continue making my own yogurt, I hope to just begin with the dregs of my previous batch and re-use my homemade yogurt as the starter. I've read that sometimes homemade starter loses its efficacy over time, but I will try this and report back in comments.)
4. Pour all that milk in the slow cooker and set it on high, using the probe, to reach a temperature of 180 degrees Fahrenheit.
4. Set about 2 heaping tablespoons of the plain yogurt starter aside in a bowl so that it warms up to room temperature. Wait.
5. When the milk hits 180 degrees, turn off the slow cooker. Stir the milk some and skim off any skin.
6. Wait, and wait until the temperature of the milk drops to 120 degrees Fahrenheit. While you're waiting, gather a couple beach towels or a sleeping bag. (In my experience, for the temperature to drop 60 degrees it takes well over an hour. It will go faster if you stir the milk occasionally. When I stir the milk, I always find more "skin" to skim off. To check the temperature, you can use a meat thermometer, but I have had better results with a candy thermometer. I think it is more accurate. If anything, I would err on starting the following step #7 in the recipe at a temperature slightly under 120, rather than over 120. If the milk is too hot, it might kill those marvelous active cultures in your starter.)
7. When the temperature is at 120, stir a little of the warm milk from the slow cooker into your starter. Stir it until it is smooth and then slowly stir all that starter+milk into the milk in the slow cooker. Stir it gently until the starter is well incorporated.
8. Put the cover back on the slow cooker and wrap the whole slow cooker in towels or a sleeping bag to incubate the milky culture. Set it aside (I like to put it in my oven (turned off!) so it is out of the way) for 8-12 hours in its warm little nest.
9. Unwrap and open up the slow cooker to find YOGURT! You'll see some whey on top. You can pour or skim that off and save it for baking or pancakes. Or you can even chug it for the probiotics, which I'll admit is kind of nasty but makes me feel pretty crunchy and strong. If you want thicker yogurt, I have had good results setting a Chemex strainer over a bowl and just letting the yogurt strain a bit. I store this in my trusty Duralex food storage containers, but mason jars, or Tupperware would work fine too.