This may be one of the easiest kitchen tricks I’ve learned in the last few years. I haven’t bought any stock since I figured this out. It saves money, but more importantly, this stock tastes MUCH better than anything you can buy.
Step 1: As you peel, chop, and otherwise use any vegetables for recipes or salads, save all the pieces you would normally throw away.
The vegetables that help the most with umami (and make your stock great) are the classic mirepox (carrots, celery and onion), garlic bits, and mushrooms. There are a few vegetables you should avoid using for stock. Some vegetables will make the stock bitter or impart a strong, very specific taste that may not work in some recipes (e.g eggplant, turnips, cilantro, ginger). If you happen to be someone who buys Parmesan cheese with a rind, those rinds are wonderful in stocks. If you use fresh herbs when you cook, make sure you throw the stems in the stock. 
Step 2: Keep a big ziplock bag in the freezer and toss the washed bits you saved into the bag. When you drain beans, tomatoes or other vegetables from cans, put the juice in the bag, too.
Step 3: When the bag is full, put the frozen vegetable bits in a big pot with water to cover them, bring to a boil and then simmer for about an hour.
If you have an Instant Pot, you can make stock in less time. I don’t add salt while making the stock because it lets me season the dishes I make to taste.
Step 4: Freeze the stock you don’t use in a day or two.
I usually freeze my stock in 1-2 cup plastic containers. Alternatively, use freezer bags if you want to take up less space in your freezer (Push the air out of the bags and lay them flat on a cookie sheet to freeze). Another trick is to freeze the stock in ice cube trays or muffin tins and then put the frozen stock in freezer bags.
In addition to recipes, use this stock instead of water when you make rice or grains. Thaw your stock in the refrigerator (if you remember) or in the microwave (if you don’t).
We eat a mainly plant based diet, so I only make vegetable stock. If you eat meat, you can save the bits from the meat or fish you cook – or ask your butcher for stock worthy bones and add them to the vegetables to make great chicken, beef or seafood stock. If you want perfect chicken or beef stock, you may have a bit more work to do… 🙂
Enjoy!
