Roux

This recipe is for classic French roux. There are lots of other varieties. Here is a link to a Wikipedia article with lots of information about roux.

Ingredients:
One stick (four ounces) butter
One cup flour

This makes a fair amount of roux to freeze and use when needed.
You may use salted or unsalted butter if you are going to clarify it.
If you don't plan to clarify it, use unsalted.

Method:
The most important thing in making roux is to not scorch the flour. The first few minutes are when the flour is most likely to scorch. If the flour is scorched throw your batch of roux away and start over. The roux will taste terrible, anything made with it will taste terrible, and there is no way to remedy this problem. However it is easy to prevent.
Melt the butter in a ten inch skillet over very low heat. Turn the heat off and fold in the flour until all the flour is well coated with the butter. Now turn the heat back on at the lowest setting available. The roux should have the consistency of warm peanut butter. Cook the roux slowly, turning it every few minutes and slowly increasing the heat as the roux cooks and the danger of scorching diminishes.
How long to cook it depends on your preferences and what you intend to do with it. It should be cooked long enough to eliminate the raw flour taste. Minimal cooking like this gives white roux which is good for white sauces. I usually make an "all-purpose" roux that is cooked longer to the blond stage. This takes about thirty minutes. Roux freezes well and can be kept for months. When you see a recipe calls for roux and yours is already in your freezer life is a bit simpler.