Basically, I always use a quart of milk. Usually 2%, but sometimes whole, because it is so sweet, you don't need extra honey. But then you have that extra fat. It turns out divine with whole, just sayin'. So, it is kind of essential to have a thermometer, preferably a candy thermometer. You don't want it to touch the side or bottom of your saucepan.
I decided to do this post after my yogurt was done, but this is actually a nice view of the candy thermometer. You want to heat your milk to exactly 185 degrees F. It takes a little bit to get to 175, but then it moves quickly to 185, so you really need to hover after a few minutes. This temperature is right before boil. If you go over that, your milk and conversely your yogurt, will have a burnt taste. So, be gentle, people. At 185, turn off the heat and let your milk cool until 115 degrees. At the same time I am doing this, I put a full teapot of water on to boil. Then I fill my thermal coffee carafes. That is where my yogurt will set. I used to just cover everything and put it next to my oven light, but it was spotty if it would set.
So, while all the milk is going, fill those carafes with boiling water. This gives the thermos a base of heat when you pour in your milk/yogurt. So, when your milk hits 115, mix in two tablespoons of yogurt at minimum, but you can use more. I use more. I generally start a batch by buying my favorite yogurt, which is usually Erivan. I just buy a little 8oz cup and add the whole thing to my warmed milk. And then I mix it with a spoon, or fork. There is no magic to it. Just add and mix. If it is lumpy, that is cool, just try to get it touching all your milk, if that makes sense. Then pour the milk into a carafe. I need to split it into two carafes. My sister does her in a cast iron pot, then covers it with its lid and puts it in the oven by her oven light, or in a just used still hot oven. Here is the thing about yogurt that used to turn me off. Let it sit for six to eight hours. Or overnight. Or for a long ass time.
In the morning, get a big ass bowl, and a big ass colander, and put them together. Line the whole thing with cheese cloth...
Then pour your yogurt onto the cheese cloth, and eh, voila, see your progress. If you are trying to do this in like six hours, check before pouring. It may still be liquid. This batch I made at 4p and this is now 7a, so it sat in the thermoses for a while.
I then spoon the yogurt into a French terrine container and store it in the fridge. I use it in a week or two. When it get to a point of being almost gone, I then make another batch using the bottom of this terrine. See how that works. One 8oz container of yogurt could theoretically last you forever. Also, you can use flavored yogurt, and the yogurt flavor or sugar will not affect the next batch.
Enjoy your next batch of homemade yogurt. YUMMY!