1.Pour the gallon of milk in a 6 qt. slow cooker. Turn the slow cooker on low.
2.Allow the milk to heat up to 180 degrees F. In my slow cooker this takes about 5 hrs. Every slow cooker is different though, so use a candy thermometer to check the temperature of your milk until it hits 180.
3.Turn the slow cooker off and allow the milk to cool to 110 degrees F. Again, use a candy thermometer to determine when it has hit the correct temperature.
4.Once the milk has cooled off to 110 degrees, preheat your oven to 200 degrees F. Remove some of the warm milk into a small glass bowl. Gently whisk the 1/2 cup plain yogurt into the warm milk in the small bowl until there are no lumps left. Gently whisk the yogurt/milk mixture back into the rest of the warm milk.
5.Pour the warm yogurt/milk into sterilized glass jars. A funnel helps with this. Put lids on the jars and place them in the oven. Turn the oven off and allow the jars to sit in there for 8-12 hours (or overnight). This is the incubation phase when the yogurt cultures the rest of the milk and turns all of it into yogurt. If your oven has a light in it, leave it on while the jars are in there. This will help to keep the oven the right temperature.
6.After the jars have set in the oven for 8-12 hours, remove them and place them in the refrigerator to chill completely before using.