Bring the milk to a simmer over medium-high heat. When it starts to steam, reaching a temperature of 180F, turn off the heat and allow the milk to cool to 110 F.
Whisk the yogurt starter into the warm milk, and transfer it to your yogurt maker. If you don't have a yogurt maker, transfer it to a glass jar and culture it in a place with a steady, even temperature of 110 F. Culture the yogurt for at least 6 and up to 12 hours. The longer it cultures, the sourer it will taste.
When the yogurt is finished culturing, transfer it to a yogurt strainer. Allow the yogurt to strain for 1 hour and then discard or repurpose the whey. Transfer the yogurt to a large bowl or to the basin or a stand mixer, and then whisk until lightened and airy - about 5 minutes. Store in the fridge, and use within 2 weeks