What do you add to boiled milk to make yogurt?

To make yogurt from boiled milk, you need to add a yogurt starter culture, which contains live active bacteria. Typically, this can be plain yogurt with live cultures or a commercial yogurt starter. After boiling and cooling the milk to about 110°F (43°C), mix in a small amount of the starter culture, then cover and let it incubate in a warm place for several hours until it thickens.