Making a roux seems like a straightforward process. It requires just two ingredients - fat and flour - which are cooked together until the flour no longer tastes raw. It's then used as a thickener for sauces and soups. But like so many other cooking techniques, there are subtleties to making a roux, and they depend on what you want the result to be.
A roux is equal amounts fat and flour, by weight. I have seen recipes that call for equal amounts by volume, but that won't work because there's a big discrepancy between the weight of fat (which is dense) and something light, such as flour: one US cup of butter or oil weighs about 225 grams while the same cup used to measure flour would come in at about 130 grams. You need enough flour to bind the ingredients into a cohesive mass because, if there isn't enough, the fat will separate and float to the surface.
When making roux on the stovetop, the ingredients should be stirred constantly. How long you cook the roux depends on what you want it for. For a white sauce - such as béchamel - the roux should be heated and stirred just long enough to cook out the raw flour taste. It will take on a little colour (especially if you're using butter as the fat) but too much will discolour the sauce. The different stages of roux are described in colours, usually white, blond and brown. In Cajun cuisine, they sometimes cook the roux until it's a very dark brown. The darker the colour, the deeper the flavour, but it also has less thickening power, and there's a fine line between a dark brown roux and a burnt one (which would taste awful). If you leave the roux in the pan it was cooked in, it will continue to darken from the residual heat even when taken off the flame, unless you immediately start adding the liquid. If you're not going to use it immediately, it's a good idea to transfer the roux to a cool metal container or pan because the metal will absorb the heat and stop it cooking.
If you want to avoid standing over a stove for a long time while constantly stirring flour and fat to make a brown roux, try the oven method. In an ovenproof pan, thoroughly combine the ingredients (if using butter, melt it first) then put it in an 180 degree Celsius oven and leave it for 90 minutes or longer, depending on how dark you want it to be. The steady heat of the oven means the ingredients cook evenly, rather than just on the bottom (as they would on the stove), so it can be stirred intermittently.
When I prepare a roux, I use a wooden spoon to stir the ingredients, but once I start adding the liquid, I switch to a whisk, which gets out the lumps. You should add the liquid in a little at a time, whisk until smooth, then add a little more liquid: if you were to add it all at once, the sauce or soup could turn out lumpy.
If pressed for time, heat the liquid until simmering before adding it to the roux, so that it stays hot.
A large quantity of roux can be made in advance, then used as needed.
Truc (tryk): noun, masculine, trick, gimmick, device. A French word for a chef's secret.