Melt Butter: place the sauce pan over medium heat and ensure the butter melts evenly - however not browned.
Make the Roux: Have the flour ready to fold into the butter. fold in slowly ans stir with the wooden spoon. Keep the roux blond.
Gradually ladle the warm milk into the roux.
Continue to heat the sauce up to a small simmer and whisk. Cook under low heat for a bit - nape' the sauce over the back of the wooden spoon. the sauce at this point should hold a line.
remove from heat and place in bain-marie for service.