We get asked this one a fair amount, so here's how we make stock in our (Kuhn-Rikon) pressure cooker.
We start by saving the veggie scraps (carrot, onion, leek, mushroom, shallots) in a produce bag. When it's full, it's time to make stock. This works out to about every 10-14 days.
In the pressure cooker, over high heat, we put 1 tsp. of oil, a couple of peppercorns, and two bay leaves.
Add the scraps and begin to saute, moving and rotating the veggies every couple of minutes. Tongs work best here, as you want to grab the stuff off the bottom and so they all get some time on the bottom of the pot.
After 10 minutes, it's dropped to 3/4 full.
After 20 minutes, it's dropped to half, and is beginning to develop a good layer of fond (browned bits) at the bottom of the pressure cooker.
We then deglaze the fond on the bottom of the pressure cooker with 1 cup of water, scraping as much off the bottom as possible.
Then we add about nine more cups of water for a total of 10 cups (2 1/2 quarts) -- this unit can hold 7 liters total, so it can take it -- but you should only fill the pressure cooker 3/4ths of the way to the top at maximum, or else it won't have enough room to build pressure, so adjust accordingly.
Secure the lid, and bring it to pressure.
The second ring on our Kuhn-Rikon equals 15psi. Cook for 40-60 minutes to fully extract all the flavor.
We drain it using a stand chinois, and really press on the solids to extract as much liquid as possible.
Yields about 8 cups of solid gold.