Step 4: Incubate
You could also try setting them in the oven, if you can set the heat that low. If you're just itching to spend lots of money, you could buy a commercial yogurt maker, which I believe maintains a constant temp as your jars incubate in a water bath. I prefer my free setup, using common household items.
That's it! Refrigerate the yogurt and it should keep for at least a month or two. You'll notice that the curd separates out from the whey after it sits awhile, so give it a stir and you're good to go. This makes a great base yogurt to blend with fruit, granola, etc. It's also good over cereal in the morning, or whipped up as kind of a yogurt drink.
Finally, you can pour the finished yogurt into a strainer lined with cheesecloth and let it hang for several hours to make a "yogurt cheese" that is much like cream cheese. You can make the plain version of this yogurt and then add any sort of flavorings to the finished cheese for a fancy spread.
Let me know what you think!