1. Clean a 1 quart (1 l) jar by pouring boiling water in it and letting it stand 5 minutes. Drain the water (carefully, as the water and the jar will be hot), and let the jar dry.
2. Heat the milk in a saucepan fitted with a thermometer, or use an instant read thermometer, until the milk reaches 180ºF (82ºC). Remove from heat.
3. When the temperature drops to 115ºF (45ºC), stir in the 1/4 cup of yogurt, then pour the mixture into the jar and cover it.
4. Put the yogurt in a slightly warm place, and leave it undisturbed for 10 to 12 hours. The longer fermentation will yield a more tart yogurt.
An oven that has a pilot light, is good. I have radiant heat (under-floor* heat) and put it in a warm spot.
5. Chill the yogurt thoroughly, at least three hours. The yogurt will thicken up once cool.