Heat milk in a saucepan over medium heat, stirring and scraping the pan, until the temperature reads 185°F.
Reduce heat to low and cook milk, without stirring or letting the heat exceed 185°F, for 20-25 minutes. (err longer to get it thicker)
Remove milk from heat and stir constantly until the temp reads 110°F. (Optionally use an ice bath to speed it up.) Immediately stir in the starter yogurt, then pour the mixture into a jar
Cover the jar and place it in a warm spot, letting it sit for 10-12 hours undisturbed. (Probably 70-77°F is your target here, colder temps will go slower)
Taste after 12 hours. If you want it thicker and tangier, let it stand another 5-8 hours. Refrigerate!