If you are using a heavy bottom stockpot, heat the milk on medium high until it reaches 180 F while stirring occasionally to prevent scorching. Always keep an eye on the milk, because once the milk gets too hot it will overboil.
If you are using a Kitchenaid attachment or a crockpot, set it to 180F
Once the milk reaches 180 F temperature keep it on the heat at the same temperature for 10 minutes while stirring occasionally to prevent scorching. You do not want to scorch it; you just need to sterilize it.
After the 10 minutes have passed, remove the milk off the heat while monitoring its heat. You want the milk to reach 115 or 110 F. If you would like, you can submerge the pot in an ice bath to speed up the process.
If you are using a kitchenaid attachment, you can remove the stainless-steel liner, submerge it in an ice bath until the milk reaches 115F, dry it off, and insert it back into the attachment base.
Once the milk reaches 115F you can now add the 1 cup of yogurt. Whisk everything for 30 seconds. Cover the pot with a lid and let it sit at this temperature for 5 up to 20 hours. You can achieve this either by covering it with a big blanket in a warm environment away from draft, or by using a heating pad.
If you are using a Kitchenaid attachment, just simply set the temperature to 115 F and set the number of hours for how long you want the yogurt to ferment, put a lid on it, and walk away.
While the yogurt ferments, you do not want to shake it, stir it or move it. All you need to do is ignore it.
The longer the yogurt ferments the sourer it becomes, and the more probiotics it has. Once you are satisfied with the length of hours that it has fermented, you can remove it from the heat, and place it in the fridge to fully cool before serving.
To create flavored yogurt, you can add a little bit of jam or preserve of any flavor into the bottom of a jar, add about a 1/3 cup of yogurt on top, cover and let it sit for a day or 2, then serve.