Making yogurt is so straightforward that the CC is surprised it's not more commonly known. When the CC was making it, two separate people commented on the fact the milk will get "spoilt".
Yogurt is spoiled milk. So is cheese for that matter. So are miso, kimchi, and all other sour pickles. It's the precision of the spoilage that matters not the fact that it's "spoilt."
Ladies and gentlemen, boys and girls, yogurt doesn't come from the supermarket. It's spoilt milk and it's important that it be "spoilt" in the right way.
Over millenia, we as a species are symbiotic with a class of bacteria known collectively as lactobacilli. (Literally, milk-bacteria.)
We co-exist and they do wonderful things for us in our guts. That's why antibiotics make you feel so queasy at the end of the dose. They wipe out all the bacteria in our body and we can no longer digest anything that well. (Incidentally, a lot of what we consider "digestion" is being done by the bacteria in our gut not by humans but's that for a different post.)
Anyway, making yogurt is dead straightforward. The only hard part in winter is setting it in a warm enough place.
4 cups milk
1/2 cup active yogurt
Heat the milk in a non-reactive pan until it comes to just under a boil. You are both killing all the other bacteria present in the milk and changing the proteins to be more receptive to making yogurt.
Cool the milk down to about 110°F (just above human temperature.) The old wives' method is to see if a finger inserted in the milk is comfortable or not. Above this temperature, you will kill all the bacteria introduced later.
Add the yogurt and whisk it into the milk. You are basically doing a precise "spoilage".
Let it sit in a warm spot for about 8 hours. It takes less time in summer and a little longer in winter.
Refrigerate and repeat.