Here's a simple and fool-proof method for making Greek Yogurt in the Instant Pot.
I'm using a half gallon of milk here because that's about the amount of yogurt my family will eat in one week. You can easily double this recipe and use a full gallon of milk. I still stick with one yogurt starter for a full gallon as well.
The yogurt culture that I recommend here can be found on Amazon or Whole Foods.