Important! Read This:

A white sauce is nothing short of miraculous for there is literally no end to its uses and variations. It is the basis for cheese sauces and creamed dishes as well as soufflés.

The thing that thickens a white sauce is flour; the thing that thins it is a liquid, such as milk, cream, or broth. One tablespoon of flour for 1 cup of liquid will give a very thin white sauce. Three tablespoons of flour for 1 cup of liquid will give a very thick white sauce. If a sauce is too thick, it is quite easy to make it thinner by adding more liquid. Add the liquid gradually, stirring, until the sauce is as thin as you want it. But if the sauce is too thin, don’t try to add more plain flour; instead, melt a little butter in a separate saucepan and then stir in an equal amount of flour and whisk until blended. Now add the sauce gradually, stirring all the time. Another way to thicken a sauce is to blend equal parts butter with flour, 1 teaspoon of each, let us say. Use the fingers to blend the two. Add this, bit by bit, to the boiling sauce, stirring as it is added. Continue adding this mixture to the desired thickness. Remember that the sauce thickens immediately after the flour is added.