Heat the milk. If you are in a hurry, 3 minutes in the microwave should do the job, otherwise heat the milk in a pan. Next, add a pinch of nutmeg.
Add the salt.
Stir.
Put the butter into a saucepan, on a gentle heat, and melt it.
Once the butter has melted and starts to bubble, gradually add the flour whisking constantly.
Whisk until all the flour is coated with the butter and the the paste achieve a sandy texture (technically, this is called a white roux). This operation should last a couple of minutes. Next, let the roux to cool down because it is not good practice to add hot milk to a hot roux (risk of burning and the mixture would become too lumpy).
When the roux and the pan have cooled down, return the pan to the fire and gradually add the milk. Meanwhile keep whisking.
Once you are sure that the roux is completely mixed with the milk and no lumps are present, turn the heat down to low and gently stir.
Keep stirring gently until the sauce thickens. Then, you can remove the pan from the heat and set aside (for a perfect smooth sauce, pass the sauce through a strainer). You bechamel sauce is now ready.