I had half a bag of frozen kitchen scraps, too much celery to reasonably eat, some carrots that were on their last legs, and some forgotten garlic cloves that weren’t going to get used.
So, I plonked it all in my biggest stockpot — that conveniently had the built-in strainer — and covered it with water. Threw some salt in. Now I’m bringing it to a boil and eventually I’ll take a look at it and see if it’s any good.
Making chicken or beef stock requires bones, the good marrow-y ones ideally, and hopefully nicely smoked or roasted. But vegetable stock? Just take the peels/scraps of used vegetables (freeze ’em up and use them later, it’s fine), and maybe some veg that you know you won’t get to eat before it’s no good, and boil and season until it tastes decent.
Don’t use starchy veg (no potato scraps) or anything you wouldn’t want to drink a broth of. I’ve got some zucchini and cauliflower that were on the border, but I can’t imagine either would taste good boiled to hell. At least mirepoix stuff — celery, carrot, onion — would be edible.