Melt margarine then add sifted flour very slowly, stirring all the time. Cook over medium heat for a few minutes until smooth, without colouring. Remove pan from heat, whisk in the milk gradually. Put back on the heat and bring to a boil, stirring constantly. Cook for 5 more minutes.
For a thicker coating sauce, use a little more margarine and flour but keep the liquid measure the same. For a thin sauce, double the liquid.
This is a good base sauce. For variety you can add parsley, chives, garlic, onion, horseradish, mustard, capers, etc.