I boil water in my dutch oven to sterilize it, stir my big spoon around in it, then pour the boiling water into my gallon thermos and put the lid on, then set it aside.
In the dutch oven I bring my “milk” (a gallon of whole milk mixed with a quart of heavy cream works great, and you have some of the mixture left over for sauces, your coffee, or whatever!) almost to a boil but not quite, stirring constantly; you heat it over medium until there are little bubbles around the edges, and it starts to steam. If it scalds at all it won’t culture (you can either make a vat of soup or throw it away if it scalds), so you can’t really do it more quickly over high heat…but it goes a little faster if you bring it to room temp first.
Remove the “milk” from the heat and let cool until a skin forms on the top. Remove that, then in a small bowl temper about 1/4-1/2 (I don’t measure) plain yogurt with a little of the warm milk, then stir it all into the pot.
Pour the extremely hot water out of the thermos, immediately add the milk mixture back into it, cap it tightly, and set it on top of the fridge for about 24 hours. I've left it up to 36 hours, and have eaten some at about 20 hours...it's all good...
It’s very easy, the most tedious part is the stirring.
(Just as a side note: before I start this I put some baking soda and some vinegar into my kitchen drain; it bubbles while I work, and then after I pour the water out of the thermos the drain and pipes are fresh and clean! Just sayin’…)
You can flavor it in your dish with just about anything, sweeten it, or whatever when you serve it. My favorite way to eat it is with a drop of liquid sucralose, a splash of vanilla, and a generous sprinkle of cinnamon.
Once you taste your own yogurt - rich and satisfying, creamy and thick - boughten will always taste like a cheap imitation!