Reconsitute milk according to the package directions.
If you are using non-instant powdered milk from food storage, you may not have the instructions. Add 3 tablespoons milk powder PER CUP of warm tap water. 110 degrees is ideal. If you want to make a stiffer Greek style yogurt, use 4 1/2 Tablespoons per cup. Since I make yogurt in pint containers, I add 6 tablespoons non-instant milk for regular yogurt and 9 tablespoons non-instant milk powder to a pint of warm water.
Add 1 Tablespoon culture to each pint of milk. Culture can be homemade yogurt from a previous batch, whey (the clear liquid that rises to the top of the container) from yogurt with live cultures from a previous batch OR store-bought yogurt containing live cultures. To start a culture from purchased yogurt, I recommend using Greek yogurt with live cultures. Other yogurts will work but they often have fewer cultures and more starchy fillers. I bought one 6 oz container of plain Greek yogurt about a year ago and have been using the excess whey from each new batch.
Use an electric pressure cooker with a yogurt setting cycle or an oven with a bread proofing setting for 6-8 hours (which will keep it at 110 degrees) OR keep it at 110 degrees Farenheit by starting with milk at that temperature and wrapping it very tightly and keeping in a warm place. An insulated container like a thermos can be helpful in keeping it the right temperature. In the summer you can use a hot car (open the window a little so it doesn’t get too hot) to keep it warm.