Add the yogurt starter, probiotic, or yogurt to the milk. Shake well in the box or pour into a quart glass jar and shake well. If in the box, transfer to a quart jar, or smaller jars if desired. No need to seal the jars but make sure that they are sterilized which you can do by running them through the dishwasher. I like to use 8 or 16 ounce Ball or Mason jars.
If you have an electric pressure cooker with a yogurt setting, this step is easy. Set the yogurt setting for 8 to 12 hours and add the jar(s). Lock on the lid, close the vent and do not check until the time is up.
If you don’t have a pressure cooker with a yogurt setting, do not despair. Find a place that is between 100° and 110°F, but no hotter. This can be your oven with the light on, the pilot light on, or set at 100°F if your oven goes that low. You can set a rack on top of 4 closed jars of boiling water in a Styrofoam or other kind of cooler, and put the jar(s) on the rack, then cover the cooler. Or even set it outdoors (on a porch, perhaps), if the temperature is close to 100°F. You can also use a yogurt maker.
It takes 8 to 12 hours for your soy milk to become cultured and turn into yogurt. Sometimes it will separate into curds and whey. You can pour off and drink the whey and keep the curds.
For Greek-style yogurt or yogurt cheese, strain the yogurt in a fine strainer or cheesecloth-lined colander for 8 hours or more. The drained liquid contains active “good” bacteria (cultures), so don’t toss it: drink it in smoothies, straight, or use in recipes that call for buttermilk-type flavors.
Refrigerate the yogurt after making and use within a week.