Yogurt is what your raw milk will become naturally if you leave it at body heat — around one hundred degrees — for several hours. Yogurt cultures are naturally present in raw milk, and thrive at temperatures between ninety and one hundred ten degrees; that is, they like it warm — they are thermophilic. To make yogurt quickly and reliably, you can add a boost of thermophilic lactobacilli (milk culture) to your warm milk; just mix a couple of ounces of regular unsweetened yogurt to your warmed raw milk, pour it into a clean jar, and set it in a warm place, like a gas oven with the pilot light on, or in a cooler with a jar of pretty warm water to keep it company, for anywhere from six hours to a day and a half.
To make yogurt:
Warm two quarts of good raw milk to about ninety five degrees — a little more or less won’t mean success or failure. With a clean fork or a whisk — it doesn’t hurt to pour a little boiling water from the kettle over all your utensils before you use them, just to give your yogurt culture less competition from other friendly but non-yogurt producing bacteria and mold in your kitchen — stir in a quarter cup of yogurt per quart, mixing thoroughly. Then pour your cultured milk into clean jars, screw on clean lids, and keep it about that warm for several hours.
That’s all there is to it!