In a slow cooker, pour in 1/2 gallon milk, cover, and set on low. After 2.5 hours, shut it off. With the lid on, let the warm milk set for three more hours.
After three hours, whisk one container (or one cup) plain yogurt into one cup of the heated milk. Pour the mixture into the slow cooker.
Wrap the slow cooker in a blanket or a couple of towels. Allow to set overnight (8-10 hours). When you check the mixture, it should be thickened!!