Pour the milk into the Instant Pot stainless steel pan. Add the plain yogurt and whisk in well.
Seal the lid. Since this is not a pressure setting, it doesn’t matter if you’ve closed the pressure valve or not. You could also use a glass lid or even put a plate on top if you prefer.
Press the “Yogurt” button until it's on the “Normal” setting and press the +/- buttons until it says 10:00 for 10 hours. You could do anywhere from 8 to 12 hours.
When the Instant Pot is done, remove the lid and test the thickness. If the yogurt is thick enough for you, then pour into a storage container (or just leave in the Instant Pot stainless steel pot and cover with plastic wrap) and refrigerate for a few hours.
If you want thicker Greek yogurt, then you have a few options. You can set up a large colander over a large bowl or over your sink and line it with a cheesecloth or coffee filters. Pour the yogurt into the colander and let it sit for an hour or two to strain out the whey. Alternatively, you can use a yogurt strainer. After pouring in the strainer or colander, refrigerate.
After the yogurt is thick enough, add your desired flavorings or sweeteners (see note) then move the finished yogurt to a storage container or individual serving containers. Refrigerate.