Add all the milk to the inner liner of the Instant Pot. Cover with either the IP lid or the glass lid (separate accessory).
Push the yogurt button until the display says boil. (This may vary based on your model)
During the boil cycle you should whisk the milk a few times. This allows for even heating.
When the beep sounds, whisk the milk and take the temperature using a digital thermometer. It needs to reach 180 degrees. If the temperature is not at 180, set it to boil again (this often happens when using an entire gallon of milk instead of half). Whisk and check the temperature every 10 minutes until 180 dgrees is reached. This is essential for getting thick, creamy yogurt.
Carefully remove the inner pot and place in a sink with a few inches of cold water in it. Adding ice to the sink will speed the cooling process. Cool the milk to about 100 degrees, whisking often.
Remove the pot from the sink of water and carefully dry the outside thoroughly. You do not want water in the housing of you IP!!
Ladle out 1-2 cups of the hot milk into a tempering bowl. While whisking vigorously, slowly pour the starter into the hot milk in the tempering bowl. Pour this tempered milk and starter back into the rest of the milk in the pot liner and whisk together.
Place the liner full of milk and starter back into the Instant Pot and cover with the lid. Press the yogurt button and adjust to the desired incubation time. Read your manual to make sure you have the right settings. The timer counts UP on the yogurt setting (which is different on other settings where it counts down).
When the incubation time is up, turn the pot off, remove the liner, cover and refrigerate for at least 6-8 hours. DO NOT STIR!
Check to see that the yogurt is set. It should jiggle like pudding and pull away from the sides of the bowl slightly when you move the bowl. It will smell like yogurt and will be thick like pudding.
Freeze 2 tbsp of your yogurt to use as starter for your next batch. If you plan on straining, you can save the whey instead.