If you can boil water, you can make yogurt. This is so easy, the Accidental Locavore makes it once a week. This makes about 1 quart.
Mix 4 tablespoons of the milk and 4 tablespoons yogurt in an small bowl and set aside. Put the rest of the milk in a large saucepan and bring to a boil. Remove from the heat and cool until it reaches 100 degrees on an instant-read thermometer (this takes about 75 minutes). A skin will form on the milk; leave it alone. When the milk reaches 100 degrees, gently nudge aside the skin and pour the yogurt/milk mix into the milk (do not stir).
Cover the pot with a clean dishtowel. Put the covered pot in the oven with the oven light on and the door closed for 16 hours.
When it's done, take the skin off with a slotted spoon (or clean fingers). Ladle the yogurt into a colander lined with 2 layers of the cheesecloth (over a large bowl, or your pot) and refrigerate. How long you strain the yogurt will determine how thick it is. For Greek style, 4 hours, for more normal yogurt an hour or 2 is fine. Transfer the yogurt into a bowl or container, serve, and enjoy.