I have some equipment and have tried this in the past with marginal success. So I started to do a bit if research on this subject to see if I could be more successful.
Issue one was Scalding Milk. Why? I found out that scalding the milk changes the protein to make it set better with the yogurt culture. Great! no Raw milk yogurt. But it is not about killing bacteria. It is about changing the protein.
What is scalding really? Most references said to "boil the milk". Or bring the milk to scalding. Or look for bubbles at the edges of the pan. Or 180 degrees F. I know that high degree sterilized milk does not get to 180 F. And the proteins are altered. So that would mean that 180f is not exact.
I have a food thermometer. I started by scalding at 170F. Worked great. Terrific thick yogurt.
Cultures. You can buy commercial yogurt with active cultures. You can buy powdered yogurt cultures. You can use some of the last batch of yogurt you made. I started with one package of yogurt culture, 1/2 cup of commercial yogurt mixed together. Great, thick yogurt. Second batch cultures were from that home made yogurt. 1 Cup yogurt for 3 quarts of milk. Take the yogurt out of the refrigerator before you start to heat the milk. Let it come to room temp. Heat milk to 170f. Skin starts to form at 140f so stirring becomes necessary. At 170f turn off the heat and take the pot off the stove. As the milk cools down it will continue to make skin. Stir often until the milk is down to 120f. At about 130f you take a 1 cup scoop of the hot milk and slowly stir it into the 1 cup of room temp yogurt. When pot of milk reaches 110 to 120f you stir the yogurt culture into the milk. Stir slowly and gently for a couple of minutes. Milk is now ready to become yogurt.
Heating and how long. I have a dehydrator that goes down to 95f I use sterilized 1 quart jars and a funnel for my canning. I measure a quart of cultured milk into each jar and cover with a press n seal plastic wrap with a canning ring. I set them on the lowest tray, not on the direct bottom, so that there is circulation all around. 3 to 5 hours, maybe more, but mine have been 3-5 hours in the dehydrator. .Some say that the longer you let it heat the more sour it gets. Also, some say that each successive batch is slower or more sour or less active or some other such thing. Yogurt is a living thing. It has its own agenda. Temperature makes a big difference but so does the social nature of mixed cultures. Be patient and look at 3 hours. Maybe need longer? don't sweat it. Let it keep going. But I would not let it go overnight. 10 hours might be too long.
Additives. I am not a fan of thickeners or gums. Some commercial yogurt has gelatin or pectin or other gum type thickeners. But I do add 1/2 powdered milk to my milk before I start heating the milk. I like the flavor and thickness without being chewy or cloying in the mouth. I do not flavor mine either. Plain is my thing.
To strain or not to strain- Greek style yogurt is strained which means you line a strainer with thin muslin like for cheese but not cheese cloth like at the store. Place the strainer over a large bowl. Pour the yogurt into the strainer. One quart of yogurt makes 1 pint of strained yogurt and 1 pint of whey. Approximately. The longer you strain the yogurt the thicker it will be. Place the whole thing in the refrigerator to drain. At this point I tie the ends of the muslin up together so they don't lay on the shelf of the refrigerator. They will leave lots of whey inside the shelf. When tied up, they leave lots of whey inside the bowl. At 2 hours check the bowl to see how much whey you have. First batch was strained for three hours and was thicker than sour cream. Second batch was two hours and very nice for lots of things. But I also like the not strained yogurt. Just so you know, strained yogurt still has active cultures and can be used in the next batch. It takes up less space in Pint jars instead of quarts.
Whey- what is it good for? Liquid for bread. Liquid to mix up concentrated fruit juice. Liquid to add to soup broth. I started using whey when I was making cheese. It is pretty useful. Keeps about 1 week like milk. It may have cultures in it but it would be rather thin. My favorite use is in fruit smoothies.