1: Melt 2 tablespoons butter in a pan on low heat. When it sizzles, add 2 tablespoons flour all at once.
2: Mix well and cook, stirring continuously, until the mixture become light brown. This mix, which can be prepared beforehand, is called a "roux", and in this case a "blond roux". If you continue cooking, you will get a "brown roux".
3: Pour 2 glasses milk (cold) onto the roux, all at once. Mix thoroughly. You have there the secret of succeeding with a bechamel:
if roux is hot, add cold milk
if roux is cold (perpared beforehand), add hot milk
Salt, pepper and add grated nutmeg. Leave to cook on low heat, stirring up continuously until bechamel thickens.
4: The thickness of bechamel is a question of taste.
5: You can make variations on béchamel, such as Mornay sauce, which is a béchamel sauce to which egg yolks and grated cheese are added.
Remarks
Bechamel can be easily made beforehand, and reheated when necessary.