Heat the milk to 180 degrees. This destabilizes the proteins in the milk which makes the yogurt thicken nicely.
Quickly cool the milk to about 118 degrees.
Stir in the yogurt with active cultures with a small amount of the milk to make sure it is incorporated well.
Add the active culture yogurt and milk to the rest of the milk, stir well.
Set it in your chosen place to keep it warm for 6 to 8 hours.