Onion skins, carrot ends, the green tops of leeks, herb stalks, a chicken carcass, the parmesan rind — all of it goes in the bin in most kitchens, and all of it is the reason restaurant food tastes deeper than home food. Stock costs nothing but a pot and an hour of doing something else.
Keep a bag in the freezer
You do not make stock on purpose. You make it by accident, once the bag is full. Keep a freezer bag and drop scraps in as you cook: onion and garlic skins, carrot and celery ends, leek tops, mushroom stalks, herb stems, parmesan rinds, the bones and skin from any roast chicken. When the bag is full, tip it into a pot, cover with water, and leave it at a bare simmer. Vegetables want about an hour; bones want three or more.
What to leave out
Anything from the cabbage family in quantity — broccoli stalks, cabbage cores, sprout trimmings — turns the whole pot sulphurous. A little is fine, a lot is not. Potato peel makes it cloudy and starchy. Beetroot makes everything pink. Anything already rotting makes bad stock, not free stock: scraps go in the freezer on the day they are cut, not after a week in the fridge. Do not salt it. You do not yet know what it is going into, and a stock reduced with salt already in it becomes inedible.
The two rules that decide how good it is
Never let it boil. A rolling boil emulsifies fat into the liquid and makes it cloudy and greasy. You want the barest tremble on the surface for the whole time. Strain it while hot and cool it fast — a shallow tray cools quicker than a deep pot. Any fat sets on top in the fridge and lifts off in one piece, and that fat is worth keeping for frying.
Store it in useful amounts
A litre of stock in one tub is difficult to use. Freeze it in portions: an ice cube tray for the spoonfuls that finish a sauce, and half-litre containers for soup and risotto. Reduce it hard first if freezer space is short. A concentrated stock takes a quarter of the room and becomes a litre again with water.
A bag in the freezer, a pot on a low flame, no salt and no boiling. Stock is the cheapest way to make ordinary cooking taste like someone knew what they were doing.