I use a combination of nonfat and whole milk powder
Mixing the milk powder with some water
Pre-heating the thermos with hot water. I use our wonderful french press, coffee making thermos that has many uses such as making yogurt.
I add hot and cold water alternately to the milk powder mixture until it is between 110F and 120F. This is the optimum temperature for the culture to grown. Then I add a yogurt starter (which is usually yogurt from my last batch), stir, put on the lid and let it sit for 8 hours or sometimes overnight. I pour off the whey and transfer it to a plastic container and store in the refrigerator.