Pour entire gallon of milk into slow cooker. Cover and turn to “High” for 3 hours or until the temperature of the milk reaches 180-185 F.
Turn off the slow cooker, remove the lid, and allow to cool to 120 F. This should take ~1 hour and 15 minutes depending on the temperature of your home. Do not allow the milk to cool below 120 degrees or else the culture will not grow.
Once the milk has reached 120 degrees, remove 1 cup of milk to a separate bowl and stir in 1 cup plain yogurt or starter. Pour milk/yogurt mixture back into slow cooker and stir to combine.
Place lid back on the slow cooker and wrap entire appliance in 2-3 bath towels to insulate.
Allow milk to ferment for at least 10-12 hours. The longer the yogurt sits, the thicker the yogurt will be.
After 10-12 hours, stir, and pour yogurt into storage container. Place in refrigerator for at least 4 hours to finish cooling before enjoying. Your yogurt will last in the refrigerator for up to 2 weeks.