Pour milk into a large, heavy bottomed pot.
Heat milk over medium heat until it reaches about 180ºF.
While the milk is warming, fill a large bowl or your sink with ice water.
Once the milk reaches 180ºF, transfer the pot into the ice water, being careful not to get any water inside the pot.
Cool the milk until the temperature reaches around 110ºF.
Once cooled, whisk in the plain greek yogurt until there are no lumps.
Pour the milk into glass storage containers (I use quart sized mason jars), cover with a lid and wrap in a clean bath towel.
You can now leave them sit on the counter overnight. I like to leave them overnight inside my cold oven with the oven light turned on. Make sure to put a sticky note on the control panel so no one turns the oven on while your yogurt is incubating.
In the morning, you should have thick, creamy yogurt. Enjoy straight from the jar or refrigerate to cool.