Heat 4 cups of milk to 180°F, then cool to 110°F
Stir in 2 tablespoons plain yogurt with live cultures
Cover and keep warm at 100°F to 110°F for 6 to 12 hours
Refrigerate the yogurt for at least 4 hours
Line a strainer with cheesecloth or a clean towel
Pour the yogurt into the strainer over a bowl
Let it drain in the refrigerator for 2 to 8 hours
Transfer the thickened yogurt to a container
Save some plain yogurt to use as a starter for the next batch