In a pan, melt butter over medium-low heat. Once melted, whisk in the flour.
Cook until combined and the mixture starts to thicken and bubble, about 3-5 minutes. If making a blond roux, cook for about 10 minutes and stir as necessary to keep from burning.If making a brown roux, cook for up to 30 minutes and stir frequently to keep the roux from burning.
Video
Notes
You may want to use vegetable oil instead of butter if making a brown roux.