At our house we make two and a half quart jars every two weeks. We are blessed here in Petaluma to be surrounded by organic dairies. The Clover processing plant is literally a few blocks away. We pick up a half gallon of organic milk at the grocery store. My friend Karen has a cow-share here in town so she's getting raw milk. I keep meaning to look into that. That would be the way to go.
So, most recipes have you heat the milk to 114 degrees. The milk we buy at the store is already pasteurized. So, I only heat it to 80 degrees just to get it warm enough to warm up the culture I'm putting in it.
I take a few tablespoons of my previous batch of yogurt and put it in each jar. Into that I pour my 80 degree milk and stir. I then put the lids on the jars and stick them in the oven overnight. We are blessed to have an old stove with a pilot light. It stays 150 degrees in the oven all of the time. I know that if you search for alternative methods of keeping the yogurt warm you will find many options. My friend Karen heats towels in the dryer, wraps them around her jars and puts them in a cooler.
It all takes 5-10 minutes to do. In the morning we have fresh warm yogurt for breakfast. I often do this in the evening before bed. It only needs to stay warm for 8 hours or so. I've forgotten it in the oven for up to 18 hours and it's just fine.
I have heard that if you use the same culture strain too many times it will eventually be unable to culture the milk fully. I have not had this happen but every month or so we end up eating all of the yogurt in the fridge and I buy a small container of plain yogurt and start over. Straus is very tart, Siggie's is sweet, Nancy's has the largest variety of cultures and is recommended following a round of antibiotics. We like the mild taste of Clover's organic yogurt so that's what we usually pick up.
I make yogurt for my neighbors and their kids like it thick so I add some powdered milk to the yogurt I'm using as a culture. It works. Yum!