You will need:
candy thermometer, another thermometer (meat or room) that goes up to 120 degrees F, a cooler, and a portable light such as a trouble light.
Sterilize everything that will touch the yogurt or its ingredients. (I rinse all with boiling water.)
Ingredients:
4 cups milk (I use 2%, but can use skim up to whole)
¼ cup nonfat dry milk
½ cup plain yogurt, must have active cultures (Stonyfield is great, with 6 cultures, but Dannon with 2 cultures works also.) (After your first batch, save ½ cup of the yogurt you make to use as the next starter.)
This makes 8 6 oz cups of yogurt. You can use up to a gallon of milk, increasing the yogurt used to ¾ cup. Since the yogurt comes in 6 oz cups, I just use the whole thing instead of measuring out ½ cup, and it works fine. I understand you can leave out the dry milk if using whole milk, but I wanted a lower fat yogurt so chose 2 %.
Begin by putting lit light bulb into covered cooler. You want the interior to get to 110 to 120 degrees. A 40 watt bulb worked for me with the lid of the cooler slightly ajar. You might need a 60 watt bulb in the winter. Keep the thermometer in the cooler, as you must check the temperature periodically once the yogurt is incubating.
Combine milk and nonfat dry milk. Heat this over medium heat, stirring, until candy thermometer reaches 170degrees F. Do not boil.
Cool pan of milk on counter, in fridge, or in dishpan of icewater until it is 110 degrees F. Stir milk as it cools. It will take about 20 minutes in the fridge, longer at room temp, and a shorter time in the ice water.
Once the milk is 110 degrees, put your starter yogurt in a bowl or measuring cup and add a bit of the warm milk. Whisk until smooth, then stir this back into the pan of milk. Pour mixture into sterilized glass cups or canning jars. Cover each jar and set into preheated cooler.
Let incubate 4 to 8 hours, checking temperature of cooler periodically. Close or open lid of cooler as needed to maintain 110-120 degree environment. Yogurt is done when it reaches a gel consistency. You can tip the jars to check. Longer incubation leads to thicker and more tart yogurt.
Chill at least 2 hours before enjoying. Will be good for a couple of weeks in the fridge.