In a large, heavy pot with a lid, carefully heat the milk over medium, stirring often, just until an instant-read thermometer reads 180°, 15 to 20 minutes. (Keep a close eye on it. You don’t want to scald the milk.) Remove from the heat. Let cool, stirring often, until the temperature reduces to 110°. Transfer 1 cup of the cooled milk to a small bowl; add the yogurt and stir gently until blended.
Place the pot with the milk in a large cooler. Slowly drizzle the yogurt mixture over the milk. Do not stir, just keep drizzling the mixture back and forth over the surface of the milk. Cover the pot. Place 2 or 3 quart-size jars filled with hot water into the cooler next to the pot to help maintain a warm environment. Set the cooler lid tightly in place. Let the yogurt rest until thickened, about 5 hours.
Remove the cooler lid, then the lid from the pot, being careful not to let the condensed water under the lid drip into the yogurt. Transfer the yogurt to jars with lids, then cover and refrigerate at least 2 hours and up to 2 weeks. For thicker, Greek-style yogurt, set a cheesecloth-lined strainer in a large bowl; add the yogurt. Cover and refrigerate, allowing the whey to drain from the yogurt, until the desired consistency is reached, 1 to 4 hours, before transferring to jars.
Note: Once you’ve made your own yogurt, you can use it instead of store-bought in this recipe, Yogurt Bowl with Dried Figs, Roasted Apples, Fresh Mint & Coconut Flakes