In saucepan, melt butter.
Once melted, add flour and whisk with butter.
Add milk to butter/flour mix and WHISK, WHISK, WHISK!
Cook on medium-high heat for 5-6 minutes. Stir often. Sauce will thicken.
If sauce is too thin, whisk in more flour.
If sauce is too thick, whisk in more milk.
Variations: Add chopped mushrooms for "cream of mushroom". Add 3-4 T chicken stock for "cream of chicken". Etc.