Pour your milk into a large stockpot.  Add 3 cups of powdered milk.

Heat over medium heat, stirring frequently.  Heat up to 180 degrees F.  Measure using a food thermometer.

Once it has reached 180 degrees F, turn the heat off and cool to 120 degrees F.

Measure your Greek Yogurt in a separate bowl and add some of your heated liquid into the bowl.  Continue to add liquid until the cold yogurt is heated to room temperature.

Add the mixture to your stockpot.  Stir until well combined with a whisk.  Then pour into your containers.  Put the lids on the containers.

Place on a tray in an unheated oven.  Put a towel over your containers and turn the light in the oven on.

Leave the yogurt in the oven for 4-8 hours.  The longer you leave it, the more tart it will get.  I left mine in the oven for about 5 hours.

Store in the refrigerator.  It will stay good for about 2 weeks.