I stumbled onto this a while ago and tried it.
3 cups of warm water (critical detail: water must not be over 105 deg F. That means put your hand in it without screaming or flinching)(I just use tap water a bit off of full hot)
2 cups of nonfat dried milk powder
1 tablespoon of yogurt with live cultures. (N.B. many, but not all, yogurts are pasteurized. They wont work as seed)
I mix the warm water and milk powder, then add the yogurt and blend with a immersion blender.
In Florida, during summer, I just leave it on the stove covered with a towel. It sets up in four or five hours.
In winter, or if you run air conditioning, It needs to go in the oven. Older ovens have a warm setting at 100 deg or so. That's ideal. Newer ovens don't go low enough. That means maybe pre-heat the oven and turn it off. Add a big pan of hot water as a heat reservoir.
The directions sometimes recommend buying "real yogurt" as starter periodically. I'm on my 4th or 5th generation with no discernible change in my product quality.
My favorite use is homemade granola with yogurt. It's breakfast and/or dessert.