It's best to use a stainless steel pan to make yogurt but enamel or glass will also work. Glass can go into the microwave and the oven.
You do not have to sterilize your equipment but it helps to make sure all cookware is really clean and rinsed well. (The extension service suggested that you wash everything in bleach water using 1 tablespoon of bleach per gallon of water.) If foreign "things" get into the yogurt, the quality will not be what you would like it to be. (If you have "redippers" (people who eat out of the container) in your family they can spoil your yogurt with one dip of a spoon with saliva on it. )
1 gallon whole milk (2% will also work but the yogurt will be thinner)
1 cup start (unflavored yogurt with live and active cultures or reserved from a previous batch or yogurt)
Put the stainless steel pan on the stove and add water to cover the bottom.
Put the lid on and bring water to a rolling boil. This helps prevent the milk from scorching and film does not form on the bottom of the pan, which makes cleaning easier. When cleaning this pan later, use cold water to clean it first. (You can use a double boiler to heat the milk if you prefer.)
When the water boils, pour in the milk. Bring milk to a temperature of 185 degrees Fahrenheit (F.) over low/medium heat. You can cover the pot with a lid. If a skin forms on the milk, remove it.
When temperature reaches 185 degrees F., remove the milk from the heat and let it cool to 120-110 degrees F. You can let it cool naturally for best results or if you're in a hurry, you can place it over a pan of cold water, in which case the texture may change a little.
When cooled, add a little of the milk into the yogurt start, then stir that mixture into the larger batch of milk.
Incubate the yogurt by putting it in a jar or other large, lidded container. Put this in a draft-free place to incubate. The environment for incubation should be fairly warm. You can 1) heat your oven to 170 degrees F. then turn it off and keep the pilot light on with the yogurt inside; 2) pour hot tap water into a cooler to 3-4" deep, then place the jar/container into the cooler; or 3) pour the yogurt into a large thermos.
Incubate the yogurt for about 3 hours. The longer you leave it, the more "bright" the flavor will be - not ruined, just more "bright."
After incubation put the yogurt in a lined colander and let sit until it drains to the consistency you like. I drain mine in the refrigerator but it drains faster at room temperature. The longer you drain it, the firmer it becomes until you get a soft cheese.
This yogurt keeps in the refrigerator about a week.
The liquid that drains from the yogurt is whey and can be stored for about a week. It can be used in baking to replace the liquid. It's excellent for use in bread.
Instant Powdered Milk Yogurt
2 1/2 cups powdered milk or 4 1/2 cups instant powdered milk
3 cups tepid water
1 quart tepid water
Incubate and check after 3 hours. (This took longer to ripen than fresh, whole-milk yogurt.)
Here is a link to another yogurt recipe and suggestions for ways to use it.