Delicious and nutritious, and potentially cheaper than the store-bought kind.
Step 1: Sanitize your tools.
Everything needs to be squeaky clean so that unwanted bacteria don't contaminate your yogurt culture.
Step 2: Bring 2 tbs. of yogurt culture to room temperature.
"Yogurt Culture" is anything in which the yogurt bacteria (usually _Streptococcus thermophilus_ and _Lactobacillus bulgaricus_) are living. The easiest place to get a yogurt culture is from some pre-existing store bought yougurt. Save 2 tbs. from your most recent store-bought tub, and bring it to room temperature while you follow the other steps in the instructable.
Yogurt cultures can also be ordered over the internet, etc. but it's more expensive.
Step 3: Bring milk just to a boil.
To make a quart of yogurt you will need 4 cups of milk. Any kind will do (yak, cow, goat, nonfat, whole, whatever.) Bring the milk just to a boil while stirring to prevent a surface from forming on the liquid. Boiling innoculates the milk, which eliminates any bacteria that might compete with the yogurt bacteria during the incubation process.
Step 4: Let the milk cool until it is lukewarm.
If you have a thermometer (a candy thermometer would work best for this), allow the milk to cool until it is somewhere between 105 and 115 degrees F. If you don't have a thermometer, let it cool to the point where a drop of milk placed on the wrist does feels warm, but not hot or burning. Stir while the milk is cooling to prevent a surface from forming on liquid. If a surface does form, skim it off and rinse the spoon with water. Try not to contaminate the milk at this point by introducing dirty utensils, etc.
Step 5: Add 3 tbs. of the milk to the yogurt culture.
One by one, add a few tablespoons of the innoculated milk to the yogurt culture. This is a sort of pre-mix that will allow the culture to dissolve more readily through the innoculated milk when it is added in the next step
Step 6: Add the the culture/milk mixture to the larger pot of milk.
Stir well once the addition is made.
Step 7: Incubate the mixture.
In order to transform from milk to yogurt, the bacteria needs some time to grow and work its magic. During this time, the mixture needs to be kept at a constant, warm temperature, ideally somewhere around 105 degrees F. The easiest way to mange this is to use a thermos, where the insulation will keep the mixture at about the right temperature for the 4 hours it needs to incubate. Before pouring the mixture in the thermos, pre-warm the container by pouring hot water in it and letting it sit for a minute. Dump the water before adding the milk/yogurt mixture.
If you don't have a thermos, you can just pour the yogurt mixture in a pan and set the pan in your oven. Don't turn the oven on... the pilot light should be enough to keep the oven at a stable, warm temperature at which the yogurt bacteria can prosper.
If you live someplace warm, or have a warm place in your house, you can just wrap the yogurt pan in a shawl or scarf or something, and leave it undisturbed. It should incubate on its own.
Step 8: Presto chango! Yogurt!
After 4 hours or so, the yogurt should be ready. One of the great things about making yogurt is that you can alter the taste of it by adjusting the incubation period. Generally speaking, the longer you incubate yogurt (or the more starter culture you put in), the more tangy the yogurt will taste.
For a nice, mild yogurt, 4 hrs. incubation is a just about right. If you incubated your yogurt in a thermos, pour it out into a container and refrigerate. If you made it in a pan, you can just move the pan into the fridge to keep it cool.
Step 9: Flavor it?
If you like your yogurt sweet or flavored, you can adulterate it after it cools. If you like vanilla flavored yogurt, don't just dump vanilla extract in, but consider making yourself a syrup (sugar+water) and adding the extract to that. Stir in the syrup to taste.