You could also place on a heating pad on lowest setting. The idea is to keep yogurt at a steady warmth for a period of 8 hours. If it gets too warm, it will curdle.
Add 3 T yogurt. Resist the temptation to add more, or you will end up with yogurt that is watery and sour. Cover, wrap in a towel and place in the oven with the light on, or the oven on the lowest setting for short periods of time. The light will act as an incubator.
If you want all the benefits of raw milk yogurt, remember to heat only to 110 degrees and no higher. This will keep the milks own bacteria alive.
Turn oven on lowest setting for 10 minutes. Turn off, but leave light on.