Heat milk slowly to almost boiling (at least 180 degrees F) to kill any native bacteria that could compete with your starter. This also changes the structure of the milk proteins to create thicker yogurt. You could omit this step if you are using raw milk, but your yogurt will be runny and may not be true to your starter culture.
Pour milk into clean quart jar(s). Cool heated milk to 110-15 degrees, thin about 1 tablespoon of starter yogurt with cooled milk then stir the thinned yogurt into the cooled milk.
Place jar(s) in a 110-115 water bath and let it ferment for about 4 hours. Cooler, longer fermentation creates a thinner yogurt that will be more tangy flavor and is more easily digested.