Pour ¼ of the milk into the inner liner (stainless pot).
Add 2 heaping Tablespoons of the yogurt starter (I like to use Fage brand). Whisk to incorporate completely.
Add sweetened condensed milk (if using), and whisk very well to dissolve and combine.
Add the heavy cream and vanilla (if using) and the remainder of the milk and whisk well to combine.
Place a lid on the pot. You can use the IP lid (on venting), a glass lid, silicone, a dinner plate, etc.
Press the Yogurt button (or dial), and set time, using the +/- button, or dial, for either 8 hours (default) or 9 hours (for a more tart flavor). Longer incubation time equals tarter yogurt.
When incubation time is finished, cover the inner liner with a paper towel, and a lid over that, and place it in the fridge to chill. Do not stir until chilled, for best results.
Remove from fridge and stir. Yogurt should be nice and thick*.