1) Place the milk in a pot and heat it over medium heat, stirring occasionally, until you start seeing some bubbles and the top is steaming noticeably, or until the temperature reaches 180 degrees F. If you have a thermometer, use it because it's easier, but if you don't have one, don't worry about it unless you are doing this as a science experiment for the kids. Yogurt is very forgiving, but the milk must be cooled sufficiently to not kill the bacteria added to the homemade yogurt from the commercial yogurt. The other temperatures used in these instructions are more flexible. The milk can go higher than 180 degrees F initially but must be cooled to 115 degrees F before the plain yogurt can be added.
2) Take the milk off of the heat and let it cool until you can hold your washed finger in it for 10 seconds without it becoming too uncomfortable, or to a temperature of 110-115 F. It cools quickly in my very cold garage. Mix the plain yogurt with a little bit of the heated milk in a separate bowl. When it’s smooth, add the mixture back to the pot of warm milk and combine well. Pour the cooled, inoculated milk into a mason jar or bowl, cover it with a lid, and place it in a snug warm place. Ideally, it should sit undisturbed for at least 5 hours at a temperature of around 110 degrees F, but don't worry about the temperature being that exact as long as the place where you're storing the yogurt is warm. If the temperature drops below 110, it's no problem, as it just takes the yogurt a little longer to set, which is good for making yogurt overnight. I place the jars in an oven set at 110 degrees F for at least 5 hours but usually for more like 8 hours during the night and it's not as sour as plain yogurt you buy at the store. If your oven doesn't stay on that low of a temperature, wrap the jars in a warm towel inside of an insulated bag with a heating pad on the lowest setting placed inside the bag between the bag and towel. The heating pad helps everything to stay warm. The longer the yogurt sits and the closer it stays to 110 degrees, the sourer it becomes. In the morning, it's perfect and ready to be refrigerated.
3) Pour off the separated whey and cool in refrigerator. It will set up a little more during storage. I like strained or Greek yogurt, so I put a piece of cheesecloth over the top of my jar and screw on the lid ring. Then I turn it upside-down in a glass measuring cup or in a mug with a wide mouth and let it sit in the refrigerator for a few hours until the yogurt's consistency is to my liking. I then replace the cheesecloth with a lid and store the yogurt in the refrigerator.
4) Use this yogurt for savory foods instead of sour cream and mayo. It makes excellent Middle Eastern or Indian cucumber yogurt. If you want to make a sweet version, sweeten the yogurt with fruit, fruit juice-sweetened jam, raw honey (not for babies), pure maple syrup, nuts, and granola. Since you can control the sourness when you make homemade yogurt, you don't need to add as much sweetener as you do with plain purchased yogurt. Enjoy!