1. Pour 1/2 gallon milk into crockpot (I used whole, unhomogenized, organic milk and a 4 quart crock). Put on lid and set on low for 2 1/2 hours. You're aiming for the milk to hit 180 degrees (this is both for pasteurization and thickening purposes- although some folks claim heating the milk to this temp is unnecessary).
2. Turn crockpot off, insert thermometer, allow to cool until milk reads 110 degrees.
3. Remove one cup of the milk. Stir in 2 tablespoons live-culture yogurt, and pour back into the crock. Whisk thoroughly.
4. Put the lid back on the crock, wrap in a heavy towel, and allow to rest for at least 4 hours, preferably overnight (8+).
5. Scoop yogurt into cups (I found these great Ball freezer jars on Amazon- they are stackable, pretty leak-proof, BPA free, and dishwasher safe) and refrigerate.
6. Enjoy! From what I've found, these should stay fresh for a week or more, but may become more sour over time. We've been enjoying our yogurt with granola and fruit, as well as in mango lassis. Yum! I can honestly say that store-bought yogurt (even my $2/carton splurge stuff) doesn't hold a candle to what I made, taste-wise. Remember to save a bit to start your next batch with.
I'm still working on tweaking some of the steps, mainly in regards to the crockpot temp. My yogurt came out a bit runny, and that's mainly because I got impatient with waiting for my milk to cool. It seems the crockpot heated the milk much hotter than was necessary. This week I'm going to try heating the crockpot on "warm" while I heat the milk on the stove. Then I'll turn off the crockpot, pour the milk in, mix in the culture, and continue with the rest of the steps.
One extra tip: Save a bit of the yogurt and don't jar it. Line a colander with cheesecloth, put a bowel underneath, and pour the yogurt over. Allow it to drain for a few hours- you'll be left with a yummy, thick, cheese-like spread that's great on toast (among many things.)