Add the flour and cook, stirring for a couple of minutes
When the mixture starts to bubble, remove from the heat
Add the milk slowly while continuing to stir until all the milk has been incorporated
Return the saucepan to the heat and stir for 2-3 minutes, until the sauce begins to thicken and just starts to bubble
Season with salt and pepper
Notes
To turn this into a cheese sauce (for macaroni cheese, for example), add 1 cup of grated cheese when the mixture starts to thicken - and do not let the mixture boil. For something extra, try adding half a teaspoon of mustard or a pinch of nutmeg.