This is the short version for 2 quarts of yogurt made at higher temps.
~1/2 gallon milk
1 cup buttermilk
2 tblsp powdered milk optional
Add powdered milk now for thicker yogurt.
Heat milk to 185°.
Cool to 130°.
Add buttermilk, stir well.
Pour into 2 mason jars and screw lids on.
Set jars in a small cooler.
Add 120°F water to just below lids.
Don't disturb for 3 hours.
I always just leave it overnight and pull the jars out in the morning. If you can get up and get it together to do it in the morning, you can take it out whenever it's set.
Some people are very conscientious about sterility in making yogurt. They boil their jars, lids and utensils. Perhaps this is people posting on the internet scared of litigation. Since my yogurt never hangs around more than a couple of weeks, and my kitchen is kept clean, I make sure my jars, lids, utensils, and pans are very clean. I don't boil them for yogurt. Anyone reading this should follow their conscience.
Here's how I do it, with pictures no less <grin>.
Dump this into a pan. For just 1 quart of yogurt use this amount of milk and 1/2 cup buttermilk. For 2 quarts, do this twice and use 1 cup of buttermilk.
Take the milk off the stove and cool it to 130°F. I put the pan in a bowl or the sink filled with cool water. Keep an eye on it. Stir the milk to make the cooling go more quickly. When the milk is 130°F, stir in the buttermilk. Stir it thoroughly.
This method is supposed to produce yogurt more quickly. Since I always do mine at night, frankly, I don't know. I simply do it in the evening and pull the jars in the morning when I get up.
This is the yogurt I use for yogurt and cucumber soup and for making cracker spread.