Dump the flour into the butter and mix with a wooden spoon or spatula. Make sure that there are no clumps of flour. Stir for a few minutes to let the flour get saturated with the butter and a little toasty. This will keep your sauce from tasting floury. By the way, what you’ve got here now, once it is all mixed, is called a roux, another cooking base, which can be used to thicken sauces, stews, and frequently shows up in gumbos. Do not panic if your roux looks more like scrambled eggs at this point than a sauce. It will all be okay.