Gather your ingredients.
In a heavy pot over medium heat, gently heat milk to 180°F (82°C). Keep between 180°F (82°C) and 190°F (88°C) for 10 minutes. Stir frequently to keep the milk from sticking.
Cool to somewhere between 110°F to 115°F (43°C to 46°C). Don’t be tempted to heat the milk over high heat, or the milk proteins will overheat and give your yogurt a grainy texture.
Remove a cup of the hot milk into a glass 2-cup measuring cup or medium bowl, and whisk in the yogurt starter. Pour back into the pot and give the mixture a good stir.
Incubate your yogurt in the containers you would like to store it in: wide-mouth quart jars, 1 cup glass jars with lids, etc. Cover loosely with the lids and place in a warm, draft-free spot. My favorite place is in the oven, with the light on. I've tested the temperature in this environment and it runs about 92°F. The ideal temperature is 90°F(32°C) to 110°F(43°C). Other good options are: an insulated cooler with a tight-fitting lid filled to 1 inch from the top of your containers with warm water (about 110°F), on top of your refrigerator, or even in a draft-free spot, wrapped in a towel and placed on top of a heating pad.
Let sit for 3 to 18 hours, depending on how warm the incubation spot is. You are looking for a set but jiggly (think jello) texture; the yogurt will continue to firm up in as it chills. See the last Note for how long it keeps, and how to preserve your starter if you will be taking a break from yogurt making.
If you are wanting to strain your yogurt for a thicker, more Greek-style version, set a colander or sieve over a bowl lined with a large coffee filter, cheesecloth, very fine sieve, or a nut milk bag and let it sit in the refrigerator for 2 to 6 hours to drain. A nut milk bag shortens the straining time from at least a couple of hours, depending on the texture you are going for, to 20 to 30 minutes. It is also reusable and each bag accommodates 3 quarts of yogurt at a time.
If you use different kinds of milk, or starter, you will have different results. Making yogurt is simple but ingredient variables, as well as trying different techniques will also affect your final product. Have fun experimenting and finding the perfect yogurt for you!
If you are happy with your yogurt, don't forget to set aside 1/4 to 1/2 cups aside for the next batch, depending how much you want to make the next time.