1. In a pot, bring the milk to almost a boil (about 3 minutes if room temperature, or 5-7 if cold) over medium-high heat.
2. When bubbles appear around the edges, turn off the stove, and cover the pot, and set it aside to cool for between 30 – 45 mins. You want the milk to be around 110 degrees at its lowest cooling point. If you have a food thermometer, you can measure the temperature that way. However, if you can keep your fingers in the milk for 10 seconds without the burning, this will also indicate the milk is the right temperature (careful!). Additionally, around this time you will want to preheat your oven to its “warm” setting, or around 110 degrees.
3. Add your live culture yogurt to the milk and whisk the mixture together.
4. Cover the mixture, and put it in the oven. Do not let the oven temperature rise above 110 degrees. Leave the mixture in the oven for 5-10 hours, until it begins to congeal. Check the yogurt periodically, and use the finger trick to check its temperature.
5. When the yogurt has congealed (when you move the pot side-to-side, the mixture will move as a unit), remove it from the oven.
6. Tie your cheesecloth or other straining implement tightly over a large bowl, making sure that there are several inches of space between the bottom of the cheesecloth and the bowl. Pour the yogurt mixture over the top of the cheesecloth, put it in the refrigerator, and let it strain overnight. The liquid whey will be at the bottom of the bowl in the morning, and feel free to discard this or repurpose it for baking or cooking. What’s left on top of the cheesecloth is your very own Greek yogurt!