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.