Sure, it's easy to stroll down the supermarket aisle and throw a cup of yogurt into your cart, but have you ever been tempted to make yogurt yourself? Here's how to craft your own brand of yogurt in the comfort of your kitchen, and get benefits in the area of digestion, improved immunity and lessened food allergies, by making natural yogurt with good bacteria probiotics.
1 quart (946ml) milk (any kind but not "ultra-high pasteurized" or UHP)
1/4 to 1/2 cup non-fat dry milk (optional)
2 tablespoons existing yogurt with live cultures (or you can use freeze-dried bacteria instead)
Heat the milk to 185ºF (85ºC). Using two pots that fit inside one another, create a double boiler. This will prevent your milk from burning, and you should only have to stir it occasionally. If you cannot do this, and must heat the milk directly, be sure to monitor it constantly, stirring all the while. If you do not have a thermometer, 185ºF (85ºC) is the temperature at which milk starts to froth.
Cool the milk to 110ºF (43ºC). The best way to do this is with a cold water bath. This will quickly and evenly lower the temperature, and requires only occasional stirring. If cooling at room temperature, or in the refrigerator, you must stir it more frequently. Don't proceed until the milk is below 120ºF (49ºC), and don't allow it to go below 90ºF (32ºC); 110ºF (43ºC) is optimal.
Warm the starter. Let the starter yogurt sit at room temperature while you're waiting for the milk to cool. This will prevent it from being too cold when you add it in.
Add nonfat dry milk, if desired. Adding about 1/4 cup to 1/2 cup nonfat dry milk at this time will increase the nutritional content of the yogurt. The yogurt will also thicken more easily. This is especially helpful if you're using nonfat milk.
Add the starter. Add 2 tablespoons of the existing yogurt, or add the freeze-dried bacteria. Stir it in.
Put the mixture in containers. Pour the milk into a clean container or containers. Cover each one tightly with a lid or plastic wrap.
Allow the yogurt bacteria to incubate. Keep the yogurt warm and still to encourage bacterial growth, while keeping the temperature as close to 100ºF (38ºC) as possible. An oven with a pilot light left on is one option; see the "Tips" section for other ideas.
After seven hours, you'll have a custard-like texture, a cheesy odor, and possibly some greenish liquid on top. This is exactly what you want. The longer you let it sit beyond seven hours, the thicker and tangier it will become.
Refrigerate the yogurt. Place the yogurt in your fridge for several hours before serving. It will keep for 1 to 2 weeks. If you're going to use some of it as starter, use it within 5 to 7 days, so that the bacteria still have growing power. Whey, a thin yellow liquid, will form on the top. You can pour it off or stir it in before eating your yogurt.
Many commercial yogurts include a thickening agent, such as pectin, starch, gum, or gelatin. Don't be surprised or concerned if your homemade yogurt has a somewhat thinner consistency without these thickeners.
Add optional flavorings. Experiment until you develop a flavor that your taste buds fancy. Canned pie filling, jams, maple syrup, and ice-cream fudges are good flavorings. For a healthier option, use fresh fruit, with or without a small amount of sugar or honey.
Use yogurt from this batch as the starter for the next batch.
You can use any kind of milk, including whole milk, 2 percent, 1 percent, nonfat, pasteurized, homogenized, organic, raw, diluted evaporated, dry powdered, cow, goat, soybean, and more. UHP, or ultra-high pasteurized milk, is processed to a higher temperature, which breaks down some of the proteins that the bacteria need to make the milk into yogurt.
All yogurt needs "good" bacteria. The easiest way to add this is to use existing yogurt. The first time you make your own yogurt, use store-bought plain (unflavored) yogurt. Be certain it has "active cultures" on the label.
Taste various plain yogurts before you get started. You will find that different kinds taste a bit different. Use one you like for your own starter. Alternatively, instead of using existing yogurt, use freeze-dried bacteria cultures (available in specialty stores), which are more reliable as a starter.
There are many methods available for incubating yogurt. Use a thermometer to make sure the temperature stays consistent. Choose the method that is most convenient and consistent for you:
Use a food dehydrator or yogurt maker with a temperature you can set.
Use the pilot light in your oven, or preheat the oven to the desired temperature, turn it off, and then leave the oven light on to maintain the temperature.
Turn your oven on periodically, as needed to maintain the temperature. This method is tricky; make sure that it doesn't get too hot.
Use the bread proof setting if your oven has one.
Place the container of yogurt in warm water in a sink, large bowl, or small picnic cooler.
Use a stove burner on low to keep a water bath warm.
Use a warming tray.
Use a crock pot or slow cooker on its lowest setting or turned off.
Use a large thermos.
Use a sunny window or a car in the sun. Note that light exposure may degrade the nutrition in the milk. 
Use warm blanket(s) to wrap your container(s).
To check the oven temperature, put a candy thermometer in a bowl of water inside the oven.
The longer the mixture incubates, the thicker and more tangy the yogurt will be.
Putting the yogurt in the freezer to cool it before to moving it to the refrigerator will result in a smoother consistency. You can also stir or shake in the lumps.
Persevere; your first batch is always the hardest.
Using a double boiler makes it easier to control the temperature.
Commercially available yogurts are usually heavily sweetened. Making your own is a good way to avoid this excess sugar.