1/2 cup live/active plain yogurt (store-bought or homemade)
Pour milk into the Crock-Pot. Cover and cook on low for 2 1/2 hours.
After 2 1/2 hours, leave the cover on and unplug the Crock-Pot. Let it sit for 3 hours.
After the 3 hours are up, scoop out 2 cups of the warmish milk and whisk it together with the 1/2 cup yogurt in a separate bowl. When it is thoroughly combined, dump it back into the Crock-Pot and stir. Replace the lid, and with it still unplugged, wrap a heavy bath towel around and over the Crock-Pot for insulation. Let it sit for 8 hours in a warm room. (Longer than 8 hours is OK too, especially if you let it sit overnight. Keep in mind though, that the longer it sits, the tangier it will get.)
*Optional: For a slightly vanilla-y flavored yogurt, add 2-3 TBSP of vanilla with the milk.
**2% milk will also work, although you will get more whey than with the whole milk.
(The Crock-Pot Lady has the recipe, more detailed instructions and pictures of the process.)