A few years ago I read of a method that didn't require the use of a thermometer and an ever watchful eye. Sign me up for that! The author said to heat the milk over low heat until small bubbles formed around the edge of the pan (180 degrees). Turn off the heat and cool until you can keep your finger in the milk for 20 seconds (110 degrees). Stir in room temperature yogurt with live cultures, 2 generous tablespoons per quart of milk, then pour into clean glass jars. Cover the jars and set in a warm place where they won't be disturbed ( I leave them overnight in my turned off oven with the oven light on) until the yogurt is thick. It will thicken a bit more after you put it in the fridge. That's it! It works every time.
One note about homemade yogurt in case you've never had it before is that the consistency isn't like store bought yogurt at all. If you add gelatin and guar gum it may be closer to commercial stuff but I'm fresh out of guar gum at my house so I just take it as it comes. Sometimes it will be a perfect creamy consistency. Other times it's runny, the batch I just made can be drunk from a glass. Occasionally it will be almost gluey. Never mind though, it's fine to eat or you can use it for cooking or smoothies. Also you can make as much as you need. I make 2 quarts at a time, my mother-in-law makes a pint at a time. Just adjust the proportions of starter yogurt for the amount you need. After you have your first bowl of creamy homemade yogurt with maple syrup drizzled over the top and maybe some pecans sprinkled on, you'll be hooked. Watch out!