The basics: Milk and a Starter. For milk, you can use skim, 1%, 2% or whole, liquid, or powdered with liquid added, or a combination of your choice that, with a little experimentation, you can perfect to your taste preferences. The lower the milk-fat content, the thinner the resulting consistency. If you are using raw milk, or milk that has been refrigerated for some days, boil it for a few minutes, then let it cool before using, as the heat would otherwise kill the bacteria cultures. It works best to add starter to milk that is at room temperature.
For the 'starter,' get a jar of yogurt from the store, taking care to check that it has live bacteria cultures, that it's plain (not flavored), and that it is not yet expired; the bacteria becomes weaker with time. If you prefer, you can use dried yogurt culture, available at natural food stores.
Stir the starter and milk together thoroughly and gently. If the starter is mixed too vigorously, or unevenly, it will affect the ability of the bacteria cultures to grow and, thus, the results. It takes about 8-10 hours to make yogurt; shorter for a thinner consistency, longer for a thicker one, but watch it, because if left in the machine too long, too much of the curd and whey will separate and cannot be reunited. It will also become more sharp-tasting when left in the machine a longer time.
Another way to make the yogurt thicker is to strain it using cheesecloth or a colander, letting it drip for a couple hours. Chill for 3-4 hours before serving. If a small amount of watery liquid (whey) develops on top, it is of no consequence; just mix it in or drain it off. Add flavoring or fruit just before serving.
Use the new yogurt as soon as possible; over time, it will become tart and the yogurt cultures will grow weak and ineffective. Plain yogurt will keep for up to 2 weeks in the fridge; less for flavored yogurt.
Cleanliness: Be sure that all utensils are perfectly clean before using them, or your outcome may not turn out as expected.
Yogurt cheese is made by straining the whey out of the yogurt, using cheesecloth or a yogurt cheese maker. It takes from 2 to 24 hours, depending on how dense you want the cheese to turn out.