Yogurt can be made from whole-fat milk or skimmed milk, by simply adding a tablespoon or two of already cultured natural yogurt and keeping it at the right temperature for the lactic streptococci and lactobacilli to become active. This must not vary more than a couple of degrees, or the bacteria become petulant and uncooperative.
The foolproof way to make yogurt at home is in a yogurt-maker, which is no more than a low-power heater with a thermostat, which keeps the milk at a constant 38-43°C/100—110°F while the culture thickens and sours the milk as it incubates. The yogurt-maker has a number of screw-topped plastic pots which sit in depressions in the heated base. These are filled with the milk and starter, then usually left overnight or for 12 hours. The night operation is preferable because you are not tempted to peek, a practice that the bacteria object to and that causes them to go on strike.