To make yogurt at home, heat 1 quart of milk to 180 degrees Fahrenheit, and turn the heat off. When the milk reaches 115 degrees Fahrenheit, mix 1/4 cup prepared plain yogurt with active cultures into the milk, and pour the mixture into a sterilized jar. Place the jar in a warm location for 10 to 12 hours, or allow the yogurt to remain undisturbed for up to 15 hours to make tangy yogurt.
To make Greek yogurt, pour 2 cups of homemade yogurt into a cheesecloth positioned over a bowl. Gather the four corners of the cheesecloth together, and twist the cheesecloth to press excess liquid out of the yogurt. After removing most of the liquid, tie the cheesecloth closed, and place the bundle in a colander.
Set the colander in a bowl, and store it in the refrigerator for two to three hours. Squeeze the cheesecloth a final time to remove excess liquid, and scrape the yogurt into a container.
If desired, flavor the yogurt with chopped fruit, such as berries, honey or maple syrup. Make consecutive batches of yogurt by reserving 1/4 cup of the prepared yogurt. Use the reserved yogurt to make each additional batch of yogurt.