Pour 1 gallon of milk (1%, 2%, or whole) into the Instant Pot. Push "Yogurt" then "Adjust" until the screen reads "boil." Place the lid on and walk away. It will heat the milk until boiling, and it takes about an hour.
Once the boil cycle is finished and the pot beeps, give the milk a quick stir and then use a thermometer to read the temperature. You need the milk to have reached 180* F. If it has not, use the "Saute" function on low to heat the milk a bit more.
Once 180* F has been reached, remove the pot insert and set it on the counter to cool to 110* F. This takes about an hour. A quicker option is to set the pot in a sink full of cold water and stir. This takes about 15 minutes.
After reaching 110* or a little lower, add 3 tablespoons of plain yogurt. NOTE: make SURE the yogurt you use as starter contains live and active cultures! Whisk to mix the yogurt into the milk.
Place the pot insert back into the Instant Pot and close the lid. Press the "Yogurt" button again and then "Adjust" until the screen reads "8:00." This means the pot will incubate the yogurt for 8 hours, or you can choose "10:00" for a tangier yogurt.
Once the incubation period completes, either refrigerate the yogurt as is, or make it into Greek yogurt.
To make Greek yogurt: strain the yogurt using cheese cloth covering a large colander or strainer. Allow the yogurt to drip in the sink or over a bowl for 1-4 hours, depending on desired thickness. The longer the strain, the thicker the yogurt.
Store yogurt in the fridge and it will be good for at least one week. Don't forget to reserve 3 tablespoons of yogurt to be your starter for next time! Freeze this in ice cube trays and you're all set.