1. In a heavy saucepan over medium-high heat, slowly heat the milk to 180°F. Turn the burner down as low as possible, and “hold” the milk at this temperature (it may climb a bit, which is fine) for 5 minutes. Remove from the stove.
2. Cool the milk to 115°F. You may wait for this to happen naturally or set the saucepan in a sink filled part-way with icy water.
3. Stir a bit of the now lukewarm milk into the 2 tablespoons plain yogurt. Scrape this milky yogurt back into the saucepan. You’ve just inoculated your milk with the starter culture.
4. Cover the saucepan and find a way to keep it warm (110°F-112°F) for 6 to 12 hours, or longer, until thick. This is the incubation phase. (You may wrap the pot in a blanket or heavy towel and set it someplace warm in your home. You may wrap it in a towel and set it in the oven with nothing but the oven light on. Or you may set the covered pot on a heating pad on the lowest setting after confirming that the temperature hovers around 110°F.)
5. Transfer the yogurt to a clean container. Chill for several hours before eating. If you like, strain off some whey to thicken (keep it!), and whisk until smooth.