Pour milk into your Instant Pot. Plug it in and close the lid. No need to set the pressure valve since no pressure will be used.
Press the Yogurt button until BOIL shows on the screen. This setting will automatically heat the milk to scalding, about 175-180°. This will take approximately one hour for a gallon of milk.
When the indicator beeps, remove the lid and don't let it drip back into the pot. Remove the stainless inner liner of milk and set it into a large pot or bowl of ice water to cool to about 110°, stirring the milk occasionally. I find that this takes about 20 minutes. Without the ice bath it will take at least an hour to cool.
When the milk cools to 110°, stir in your yogurt. Place the pot of milk back into the Instant Pot and return the lid as before. (If the temperature is higher than 115°, you will likely kill off the live culture. No worries if the temperature goes below 110°.)
Press the Yogurt button on the Instant Pot until the YOGT setting shows. Set the timer for 9-12 hours. I prefer 12 hours. This setting will gently heat your milk and hold it at the proper temperature for the desired length of time. The time shown will count up.
At the end of the timed fermentation cycle, set a colander in a large bowl. Be sure that there are several inches of clearance so the whey will drain away from the yogurt. Line a large colander with a squeaky clean thin cotton tea towel or clean piece of muslin cloth. Pour or ladle your newly made yogurt into the lined colander carefully. Let drain for an hour or two, regularly scraping yogurt from the towel so the excess water continues to drain. About every 15 minutes, I go around it with a rubber spatula, scraping from the top rim to the center, as if I was loosening slices of pie.
When the yogurt is a little thinner than you'd like your finished yogurt to be, remove the colander and pour off the whey from the bowl. (This is acid whey and doesn't have as many uses as sweet whey.) The yogurt will thicken as it chills.
Pour the yogurt into the bowl by collecting the corners of the towel and deftly dumping it out, leaving room to pour and the towel in it as little as possible. Slowly raise the top edge of the towel, scraping the yogurt into the bowl. (Place the towel in the sink and give it a thorough rinsing before laundering.)
Whisk your yogurt thoroughly for a minute or so and it will become smooth and shiny. Portion it into containers and refrigerate.
Your yogurt will keep 10 days to 2 weeks in the refrigerator.