Using a large pot, heat the milk to about 195-200 F (~91- 94 C). This is for two reasons:. One: It kills any other bacteria that might be in the milk that would compete against the bacteria that convert milk to yogurt. Two: It changes the milk protein in a way that allows it to culture and firm up.
Let it cool down to 110-115F (43-46C).
Wisk in the yogurt culture into the milk gently.
Transfer into the yogurt jars and close the lids.
Wrap jars in a thick blanket or put in a ice cooler isolated with a blanket over night.
After 6-8 hours the yogurt should be firm. Test by gently turning it to see if it keeps its shape. Transfer jars to the refrigerator.