Place chicken carcasses into instant pot. Make sure to keep below "fill line" in pot. Add carrots, celery, onion, and garlic in and around carcasses. Place herbs and seasonings on top of carcasses.
Add water to cover chicken bones staying below fill line.
Place lid on and lock into place. Turn the knob to "sealing". Push the "Soup" button and change the time to 3 hours. The until will come to pressure and count down the 3 hours. Let the unit then do a natural release which should take about 45 min to an hour. Make sure pin has dropped before opening lid.
Strain into a large bowl, pressing down on the ingredients to release all bone broth.
At this point you can fill jars or place in zip loc bags to freeze.
