I'm afraid I sort of cheat. Years ago I indulged myself and bought a simple electric yogurt maker in Lakeland and I've used it ever since. It has earned its cost over and over again since then.
This is it.All you do in mix a litre of long life (I use semi skimmed because it is less than 50p in Aldi and full fat, although it makes gorgeously creamy stuff, is nearly twice the price) with a few tbsp of the previous batch of yogurt as a starter in the inner container. Put on the lid, put it into the outer container, put on that lid and turn it on. About eighteen hours later you have a pot of lovely, home made yogurt.
You can do the same but keep it warm in other ways - such as gently heating the mix a little and pouring it into a flask.
So that's it. How I make yogurt.