Making yoghurt from fresh milk requires first scalding the milk to kill any organisms that might be in it, then cooling to body temperature before mixing in the yoghurt culture. The process is time-consuming and can be messy too (the milk can suddenly bubble up out of the pot when it starts to boil--even if it does not, the pot can still be a bit messy to clean). My quick method takes advantage of the relative absence of organisms in properly stored instant non-fat powdered milk.
container with lid for making the yoghurt: I usually re-use a 32-oz (1 liter) plastic container that originally held commercial yoghurt purchased in the supermarket.
source of warm heat: I used to keep the fermenting yoghurt warm on the top surface of a gas range near the pilot light. When I no longer had a stove with a pilot light I had to find another warming source. I found a cylindrical table lamp that uses a small, flame-shaped (for chandeliers) 40W incandescent lightbulb in the base. The cylinder is a metal mesh, so, after I removed the translucent white plastic inner cylinder, there is plenty of ventilation on the sides, and then I place a thin aluminum take-out container/dish on the top, and then place the yoghurt container on top of that. The idea is to keep the fermenting yoghurt at about body temperature. (If the temperature is too low, the yoghurt will ferment too slowly. Too hot, and the bacteria will be killed.)
small towel or dish towel to wrap over sides and top of container for insulation to keep warmth in.
kettle or pot for boiling water.
You can also buy equipment especially made for making yoghurt (for example, a Yogourmet, which can be purchased from Lucy's Kitchen Shop.)
instant nonfat dry milk. I generally buy a large (64-oz = 1.8-kg) box that contains enough for 20 quarts (liters) of milk.
water. (I filter my water with a Brita filter.)
yoghurt. Commercial plain yoghurt, initially, with active yoghurt cultures/bacteria—choose one you like (or a couple different brands and use a bit of each). Once you have made yoghurt successfully, use some of what you made to make the next batch, and later use some of the second batch to make the third batch, and so on.
Put the amount of dry milk in the container that you would use to make a full container of milk (generally 1 1/3 cups dry milk for a 32-oz container). Have a pitcher of cold water and a kettle of boiling hot water in reach. Add a bit of each (more of the hot than the cold) to the dry milk to make the container about half full, and immediately stir well with a large spoon to dissolve completely the milk. Then add a bit more cold water, stir, and feel the outside of the container with your hands. It should feel about body temperature. If it is too hot, add a bit more cold water; too cool, add a bit more hot water. Continue in this manner until the container is almost full and feels about body temperature.
Now mix in about a tablespoon of commercial yoghurt, or of your previous batch of yoghurt. (I usually skim off a layer to eat first, then use yoghurt that is below, in order to minimize introduction of extraneous airborne organisms that may have landed on the old yoghurt into the new batch.) Stir very well (at least about ten times clockwise and ten times counterclockwise). Then put the lid on the container.
Now place the container on the warming surface, and wrap the sides and top of the container with the towel. Leave undisturbed for several hours. As little as 4-6 hours is enough for the bacteria to congeal the milk into yoghurt. I like to leave it for 12 hours. It then has a nice sourness to it. According to an alternative medicine website, if you let the yoghurt ferment for 24 hours, then all the lactose sugar in the milk will have been consumed by the bacteria, and you should then have a lactose-free yoghurt.
From the warming surface, you can put the yoghurt directly into the refrigerator. There it will keep well for at least 2-3 weeks.