Add the yogurt and optional ingredients to the milk and use a clean whisk to incorporate the ingredients into the milk.
Close the Instant Pot lid and set to Yogurt mode for 9 hours. See blog post for instructions on each model.
Press Cancel and open the Instant Pot. If the yogurt appears to be thickened, use a clean teaspoon to take out a small amount of yogurt and see if it's thick enough and tart enough for you. If it isn't, put the lid back on and let it run through the Yogurt program for another 2 hours.
I used the full-fat variety of Fage brand of yogurt.
Remember to save a few teaspoons of your plain yogurt for the next batch.