Heat milk to 180°F to 200°F
Cool milk to 110°F to 115°F
Stir in plain yogurt with live active cultures
Mix gently until fully combined
Pour into a clean container
Cover the container
Keep warm at 100°F to 115°F for 6 to 12 hours
Check for desired thickness and tanginess
Refrigerate for at least 4 hours
Save a small amount for the next batch