whisk in flour and cook until all flour is absorbed
slowly pour in liquid as you whisk it into the flour and butter mixture
alternatively you can add in 1/4 cup liquid at a time and whisk until each addition is absorbed before adding more liquid and repeat until all the liquid is added
Notes
you can use any type of milk, whole milk will give you a richer tasting sauce, but if you are watching what you eat, skim milk does make a great sauce. Don't be fooled into thinking a sauce has to be full of fat to be good.