Let me just say that I LOVE my 8 quart oval electric pressure cooker, and I use it almost exclusively for making stock. and cheesecake. (That's another recipe for another time...) It is so easy and cost effective, not to mention better tasting and better for you - I can't imagine going back to canned broth! I started by following Lorna Sass's recipe to the letter - until I realized that you can't really make a "mistake" making stock. I got tired of throwing away scraps every day, and tired of buying fresh veggies just for stock.
I have included step-by-step photos to show how little waste there really is. Basically, over time, I put all of my veggie scraps into a gallon zipper freezer bag - onions (with skins), carrot peels and tops, celery leaves and ends, and garlic peels and ends, and parsley stems. I've tried adding veggies like mushrooms, but this yielded too strong a flavor, so I quickly went back to the basics. When the bag is full, I make stock. Usually, the timing is good - by the time the bag is full, I'm almost out of stock anyway. If I've used bone-in chicken in the meantime, I'll save those scraps, too, in a separate bag.
When the time comes, if I feel the ratio of veggies is off, I'll add a little more of one or the other - usually, I add some extra celery leaves, as I don't use as much celery as I do carrots and onions, for example. But whatever - like I said, you really can't mess this up.
Because I'm using an 8-quart cooker, and filling it to the max, I get a LOT of stock! Plus, by starting with a whole roasting chicken, I can usually harvest about 4 cups of cooked chicken meat to use in other recipes as well. Considering I can get a roaster for 75 cents a pound, and everything else is scraps, that's quite a bargain!!