Pour 1 gallon of whole milk into the Crock Pot. Place lid on and turn to LOW for 2-1/2 hours.
After 2-1/2 hours, turn Crock Pot off and unplug it. DO NOT take the lid off, just let it sit to cool for 3 hours, undisturbed.
After 3 hours, gently stir the 1/2 cup of plain, yogurt with active, live cultures into the warm milk. Place lid on the Crock Pot and wrap a large towel around Crock Pot and leave overnight or 10-12 hours.
In the morning, take the lid off and you have yogurt. Ladle out 1/2 cup of yogurt and put it in an airtight container in the refrigerator to use in the next batch of yogurt you make.
***To make thicker Greek yogurt*** Line a colander with coffee filters or cheese cloth and carefully pour yogurt into the colander placed in a bowl and let drain for 8-24 hours, covered in the refrigerator. The longer you let it drain, the thicker it will be come.