8 cups (1/2 gallon) milk
1/2 cup store-bought live/active plain yogurt
Turn slow cooker on low. Add milk. Cover and cook on low for 2-1/2 hours.
Unplug slow cooker. Leave the lid on, and let it sit for about 3 hours.
Scoop out 2 cups of the milk(should be just above room temperature) and put it in bowl. Whisk in yogurt and put the mixture back into the slow cooker. Stir to combine.
Put the lid on, keep it unplugged and wrap a heavy bath towel around the pot for insulation.
Let it sit for about 8 hours, or more.
Chill in a plastic container in the refrigerator
Source: A Year of Slow Cooking, crockpot365.blogspot.com