1. In a large pot, heat the milk until it reaches at least 180° F, stirring occasionally.
2. Once the mixture reaches 180° F, remove it from the heat and allow it to cool to 110° to 115° F. You can speed the cooling process by plunging it into an ice bath. In the winter I just put the pot outside to cool.
3. Combine one cup of warmed milk with your yogurt and stir gently to combine. Add this mixture back to the large batch of heated milk.
4. For the next 5 to 10 hours, depending on desired flavor and consistency, keep the yogurt between 110° and 115° F. You can get creative with how you do this. As I mention above, I use a crock-pot with the thermometer tucked along the side. You could also use a thermos or an oven with just the pilot light on. They key is to check it often. The longer you incubate the yogurt, the thicker and tarter it becomes. Do not stir the yogurt during incubation.

For regular yogurt: Cover the yogurt and refrigerate at least two hours before eating or straining, ideally overnight. If whey (liquid milk protein) separates out, just drain it off or stir it back in before eating. To make Greek yogurt you have to strain it (see next step).

For Greek yogurt: Greek yogurt is thicker and creamier because it is strained to remove the whey before eating. This can be accomplished in a number of ways. I have a fine mesh strainer that I load up over a mixing bowl in the fridge overnight. You can also strain it through a couple layers of cheesecloth or even two paper coffee filters taped over a small bowl. No fancy equipment needed, but if you want something specifically for making Greek yogurt you can buy a Greek yogurt strainer (link in the ingredients section). If you overstrain your yogurt just stir some of the whey back in.