Etymology is interesting.
For instance, the word "stockpile," while not generally used in this sense in contemporary English, clearly has roots in the concept of an overflowing pile of vegetable scraps used for making stock.
A stockpile is easy to build. It's kind of like a compost heap, except that you don't want your scraps to decay. Instead, you want them to take a nice non-engooening nap until you are ready to make the stock. So when you are making something involving vegetables, you take your trimming pile and put it in the freezer. Repeat scrap scavenging until pile is large and delicious.
You can use practically any vegetable except cruciferous business, like broccoli and cabbage; they reek when boiled. If you have fresh herbs, the stems are particularly good. Parmesan or other hard grating cheese rind is interesting and makes the stock heavier bodied as well. My usual pile includes a ton of onion and garlic bits, spinach stems, and maybe some potato peels. Anything else is just bonus.
This also illustrates just how cheap you can be while still being a sensible taste-oriented person: you are essentially saving your trash (from the cheapest kind of food to buy, besides) in order to produce delicious foodstuffs.
Eventually you get to the point where you want stock. Then get out a huge handful of scraps, stick them in a pot of water, and boil for a half hour or so at least. Sift out the vegetables and press them to get out all the liquid. That is it. You are done.
You are also smug. Smug! There is possibly nothing more food-smug than actually keeping a stockpile. It's worth it, though, unlike other smug things such as balsamic vinegar.