Fill a 2 qt. Pyrex batter bowl with milk. I use the microwave to heat my milk to a temperature of 175-180 degrees. I use 5-6 minute intervals for a total of 17 minutes. I stir the milk between each e interval to reduce the "skin" that forms on heated milk. Do not skip the heating process. It is important to unravel the proteins so they will behave during the incubation process.
After heating, allow milk to cool until temperature drops to between 110 and 120 degrees. This can take 30-45 minutes. I use a cooking thermometer to check. You can cool it faster in an ice bath, but I typically just let it cool on its own.
When the milk reaches the desired temperature, whisk in 1-2 teaspoons yogurt as a starter from your favorite brand of plain yogurt (but it must contain live cultures and should not have any additives). After you make the yogurt the first time, you may use yogurt from a previous batch to make the next.
Cover milk with cover or plastic wrap and place in a warm place to maintain a 100-110 degree temperature. I wrap my bowl in a bath towel and place in the oven. I turn the oven on for about a minute to warm it up (don't leave it on unless you have a 100 degree setting!)
Yogurt needs to incubate for 6-10 hours. The longer it remains, the more tart the flavor will be. I prefer mine after only about 6 hours.
After incubation, I pour the yogurt in a strainer that is lined with cheese cloth. This allows for the whey to drain off (thus Greek style yogurt). The whey can be saved and used in most recipes instead of buttermilk. I allow the yogurt to strain overnight in the fridge. The next morning I take out 2 teaspoons to save for the next batch and store the remaining in a container.