I use my grandmother's rule: 1 tbl butter to 1 tbl flour to 1 cup milk. Melt the butter over low heat, stir in the flour and remove from heat. Add milk a little at a time, stirring and mashing out any lumps with the back of your spoon. Return to heat, turn to med/high, and cook, stirring constantly, until the sauce reaches the desired thickness. Remove from heat and stir in any desired seasonings.
We get by with a little help from our friends