Pour 1 full gallon of milk into a large pot and set over medium heat uncovered.
Place a thermometer into the milk and whisk frequently to heat it evenly and so the bottom of the pot doesn't scorch.
Let the milk come up to 185 degrees. It NEEDS to reach this temp if you want it to work!
Remove milk from heat and let it cool to 110-120 degrees.
Whisk in 1/3 cup room temperature plain greek yogurt.
Cover pot with tight lid and either wrap it well in towels or blankets or place it in a warm spot in your house. It needs to stay warm and undisturbed for 10-12 hours so the yogurt can work it's magic and turn that gallon of milk into yogurt. Cathy placed hers on her stove with the oven light on for a small heat source.
After 10-12 hours, milk should have thickened and turned into yogurt. Pour off any excess liquid and then scoop yogurt into a cheesecloth lined strainer. Place into refrigerator until yogurt is the desired consistency you want. This will continue to remove they whey still left in the yogurt.
Spoon yogurt into air tight containers and keep in refrigerator.