Stock is the foundation of many dishes, so it's imperative to use a quality one or risk writing the whole dish off. You can buy stock, but generally the quality available is pretty poor, so you're better off making your own and freezing batches to have on hand when you need it. If you are buying stock, you may want to correct the flavour and mouth-feel before adding it to your dish. You can do this by adding more vegetables and meat scraps (browning the scraps first if using in a brown stock). Bought stocks don't contain a lot of gelatine - which comes from the bones and creates a jelly-like texture when chilled. Adding a pig's trotter to a large quantity of bought stock can increase the viscosity - this will help produce the desired sleekness in a finished sauce. Always taste your stock and make sure you are happy with the flavour before using, as you can't change it once added.
If you're going to the trouble of trying to correct a pre-made stock, then you may as well make your own from scratch. In general, a stock will simmer gently for a few hours, so there really isn't much legwork on your behalf, just pop it on the stove and leave it be. But first, there are a few important things to consider when making a stock.
With white stocks, it's important to rinse the bones under cold running water to remove any blood. This isn't necessary with brown stocks as the blood will cook and congeal before being added to the pan. Remember your stockpot isn't a garbage bin, so don't throw just anything into the stock. However, if you have any tomato and mushroom scraps you can add them to brown stocks to enhance the flavour.
Add enough water to the bones and vegetables to just cover them. The water must be cold to start with before bringing it up slowly to a very gentle boil. Once it is at this point, reduce the heat so the stock is only just bubbling away and cook for as long as it takes to infuse the water with the flavour. Fish and vegetable stock will take around 1-2 hours, white and brown chicken stock about 4-6 hours and veal, beef and lamb stock about 6-8 hours. The heartier the bones, the longer it takes to withdraw the essence. Once bubbling, it's important to occasionally skim any scum that rises to the surface. Apart from the skimming, leave the stock to gently tick over. If the heat is too high and it returns to the boil, throw a few ice cubes in to drop the temperature.
When the stock is ready, the straining is just as important. If you strain the stock, make sure to remove as much as you can first with a ladle, then scoop out the bones and vegetables with a spider or wire strainer, and discard, straining the remainder of the stock through a muslin-lined fine sieve. You can find muslin at most kitchenware stores. It can be a little expensive, but can be washed in some soap-free hot water, dried and re-used for your next stock. Muslin is useful as it's a fine gauze, therefore catching fine sediment from your stock and leaving you with a clean result.