Place 1/2 gallon of milk in Instant Pot and add lid. The venting lever can be open or closed, it doesn't matter.
Press 'Yogurt" button until the display shows "BOIL" The IP will bring milk to proper temperature (This step takes about 25 minutes.)
IP will beep when the milk has reached approximately 180 degrees. Test with instant-read thermometer. (If making sweet yogurt, add sugar now and stir well to dissolve.)
When 180 degrees has been reached, remove pot & place on counter to allow milk to cool to approximately 100 - 105 degrees Fahrenheit. This step takes about an hour.
To speed cooling, fill sink or large bowl with ice water. Then place the pot with milk in the cold water.
When milk is at a safe 100 - 105 degrees, add yogurt starter and stir well to incorporate. (if making sweet yogurt, add vanilla now).
Yogurt will firm a little more upon cooling, although it's a more silky yogurt than the ones in the store since they add gelatin & thickeners to the commercial stuff