Milk (About a gallon, but you may use any amount)
Half a carton of plain yogurt (About 8-16oz)
Slowly bring milk to a light boil
Turn heat off and let cool (If you can stick your finger in the milk to the count of 11, it should be cool enough)
Vigorously mix the yogurt into the milk
Let the yogurt sit out under a blanket for at least 12 hours
Refrigerate and serve!