Take quart of milk and scald by heating to 180°F. Allow milk to cool to between 105-115°F. Add ½ cup of plain yogurt with active cultures and stir.
Pour milk into sterilized quart jar and place in a cooler filled with warm water (like 100-110°F water.) Let sit for 5-7 hours. Once the yogurt is set, save the first half cup in a separate jar for starting the next batch.