I prefer the texture of a gravy starting from a roue, rather than a cornstarch gravy, which is also easy, but a wire whisk is a must.
I'm usually just making it for two in a small saucepan, so I start with melting 2 tablespoons butter, whisk in 2 tablespoons flour, and let it cook for a couple of minutes until nice and bubbly.
Add one cup of chicken or beef stock, 1/4 cup at a time, whisking well between each 1/4 cup. Season to taste with salt and pepper. Sometimes I change up the flavor with a dash of wine or sherry.