Very simple and straightforward. I use a Thermos flask nowadays and, once it’s made, decant it into a couple of jam jars.
I’ve found that the best thing to do is to use whole (full fat) milk; a pint plus a third to a half of another pint, including the cream. Rinse a saucepan, because using a wet pan helps to stop it sticking, and bring the milk to the boil. Take it off the heat and let it cool down to 120ºF/49ºC. You can put the pan in a bowl of iced water, but I don’t usually bother. While the milk is heating, I pour some hot water into the Thermos, leave it for a few moments and pour it out again, just so it isn’t cold when I put the milk in.
Once the milk has cooled down to the right temperature, I put a spoonful of yoghurt into the Thermos and add a little of the milk, stir it (using the handle of the wooden spoon) and then add the rest of the milk, in two or three lots, stirring as I go. Then I put the lid on the flask and leave it for at least 6 hours.
If I’ve used enough creamy milk, then I can pour it into the jars and it’ll be perfect. If I’ve been a bit short and had to use some skimmed milk, then there’s some visible whey. In that case, I just tip it carefully into a sieve (over a bowl, to be on the safe side) and discard the surplus whey (or soak it in bread for the chickens). In that case, you’ll get a bit less yoghurt and it might have a slightly granular texture, but it’ll taste the same.
I suspect it’d be fabulous with Jersey milk or a carton of single cream, but I haven’t tried. Every few batches, you need to start with a spoonful of bought yoghurt.
I tried, once, just to heat the milk up to the required temperature, rather than boiling it and letting it cool. It didn’t work. It just didn’t turn to yoghurt. I don’t know the chemistry, but boiling it makes the difference.