Mix water and soy powder in a saucepan that holds at least 8 cups. (If you are not careful, this mixture can boil over so you want to use a larger pot than might seem necessary.)
Bring to just boiling and whisk.
Allow milk to cool to 110 degrees F (takes 20-30 minutes depending on the temperature of your kitchen)
Whisk in pro-biotics and/or a tablespoon of already-made yogurt.
Pour into five small mason jars and close lids.
Place jars in a small cooler along with a mason jar of boiling water.
Close cooler and check yogurt after 3-4 hours.
If not set yet, refresh boiling water and check again in a couple hours.