Next make the sauce. Using the same pan first put in the butter or margarine and gently melt, stirring occasionally. Then add the flour and mix together to form a roux. Keep on the heat, stirring all the time, for 4-5 mins to cook out the flour. Then add the milk a little at a time - stir well and allow to heat through each time you add some so that it thickens. This whole step usually takes me about 20 minutes.