Once butter is melted and stir in flour. This should be a blonde roux, making sure the color is light.
Once you have a blonde roux, add all the milk at once.
Turn the heat up just a little and using a whisk stir the roux, continue to stir until the roux comes to a boil. You will know the Béchamel by placing a spoon in the sauce and then run a line down the back of the spoon. If the line holds the sauce is ready to be seasoned to your liking.
Kill the heat to the stove and season the Béchamel to your liking.