First, pour the milk ( I usually make 2-4 L, or 1/2 to 1 gallon, at a time) into a large pot, and set it on medium heat. You want to bring the milk to a boil, but slowly, so that you don't burn it on the bottom of the pot (it's very easy to burn milk). Towards the end, watch it very closely, as once it hits the boiling point, the foam on the top will immediately start to rise, and if you don't quickly remove the pot from the element, it will boil over the top (all over the stove, all under the element, very difficult to clean). I know this well, because I do this about (half of the time)
% of the time I make milk, because I so often forget about my milk while I am doing other things.
Once the milk has just boiled, remove it from the heat and allow it to sit and cool off. Depending on how much milk you are doing, this can take up to a couple of hours. I have put my milk in the freezer to cool when rushed for time. You want it to cool down until it is just warm (so that you could comfortably put your finger in it). Honestly, I'm sure there's a specific temperature, but I never bother with thermometers. I just wing it, and it works, so why make it more difficult than it needs to be?
Next, set aside several clean, glass jars (canning jars are perfect, but so are old mayonnaise or pickle jars). Get out your yogurt starter, and put about 1 large tablespoon per litre (or quart) or milk into one of the jars. Then add a bit of the warmed milk, and mix it until the yogurt is dissolved into the milk. Pour this mixture back into the pot and stir well. Now, you can use a ladle to fill up your jars with the milk mixture.
Turn on your oven light (just the regular light that you use to peek in on your cookies while they're baking). Put the jars full of milk inside of the stove. I usually cover them with a piece of paper towel or a clean dishcloth. Leave them in the oven for 6-8 hours. I prefer to put mine in at night, before bed, as it is ready in the morning when I get up. Today I made mine at about lunchtime, and it was ready just after dinner.
That's it! Today's yogurt, ready to be put in the fridge to cool off and be eaten for breakfast tomorrow!