It's really easy (and I messed it up really badly the first time, so don't give up after the first try)
So many different ways to do this, but the best-best way for me is as follows:
prep: double-boiler, so outer pot fill 1/3 with water, that is, high enough for medium-sized inner pot to fit into the outer pot and have the water up to the lip of both. Half-gallon of organic, fat-free milk, yogurt starter (any probiotic yogurt will do, I use 'siggi's skyr'). Yes, you need yogurt to make yogurt.
Turn your oven light on now, which is late evening, after supper.
- Bring water to a rolling boil with the inner pot inverted over the outer pot. This sterilizes the inner pot.
- Seat inner pot into boiling water, pour milk into inner pot, bring heat down to a light boil, heat milk this way for 30 minutes (it will not boil), bringing and maintaining milk temperature at 180 deg F. (it just does this, some say use a thermometer, but there's no need to fuss over this)
- remove from heat, empty outer pot of hot water, then, in your sink, refill outer pot with cold water, stirring the milk and bringing its temperature down to 120 deg F, which takes 5-10-15 minutes of stirring. Don't overcool it, as it slows the fermenting process too much. You'll get a feel for this eventually.
- 'Pitch' the yogurt. That is mix a tablespoon of yogurt with a half-cup of milk completely, then pour that mixture into the heated milk.
- Cover the pot of milk with a towel, place in oven next to your oven lamp, close oven.
- Walk away.
- LEAVE A NOTE ON THE OVEN FOR YOUR DAUGHTERS NOT TO PREHEAT THE OVEN FOR THEIR LATEST RECIPES! (lesson learned the hard way)
- The next morning, the yogurt is ready if you like runny-silky. That's fine for most.I make skyr, however, which is Icelandic yogurt/cheese, so:
- Place a cheesecloth (I use a sturdy cloth napkin) over a strainer big enough to hold the yogurt, place strainer over big, outer pot, and pour the yogurt into the (cheesecloth-covered) strainer. Stow in the refrigerator. Walk away.
- The next morning, you have skyr above the strainer and pure whey below. You can discard the whey or make whey-berry-soda if you'd like, but that's another recipe.
* need, v: 'need' as in, yes, eventually the milk will generate the bacteria that will consume the sugars and produce yogurt, but an infusion from already-made yogurt gets things moving along very nicely (like, the fermentation-process just takes overnight, not days).