Heat milk to 180°F to 200°F
Cool milk to 110°F to 115°F
Add 2 to 3 tablespoons plain yogurt with live active cultures per quart of milk
Whisk gently until fully mixed
Pour into a clean container
Cover the container
Keep warm at 110°F to 115°F for 4 to 12 hours
Check for desired thickness and tanginess
Refrigerate for at least 4 hours
Save a small amount for the next batch if desired