In a saucepan melt the butter (on a low heat, taking care not to brown it). Add the flour and combine using a whisk. Once the mixture has thickened gradually add the milk stirring with the whisk all the time. You can increase the heat a little to speed things up, but whisk continuously to prevent lumps from forming and the sauce from sticking to the bottom of the pan.
Once all the milk has been added season and simmer gently for about 3 minutes, stirring all the time. If you don't use the sauce immediately it will thicken after a while. Just add a bit of milk/water when you are ready to use it and heat it up gently stirring all the time (you may have to adjust the seasoning too).
Use a whisk, rather than spoon, to make the sauce in order to ensure it's smooth.
Add the milk gradually whisking all the time.
Season according to preference - see suggestions in post.
Reheating: The sauce will thicken after a while so whisk in a bit of milk/water to loosen it when you reheat it.