Sources of fat vary. Some recipes call for oil, some use butter, others use rendered meat fat.
using different fats will result in different types of Roux. A light Roux does not add much flavor to what it thickens but it thickens very well. A darker Roux will lend more flavor to a dish but will not thicken as well as a light Roux.
Darker Roux will be made using oil instead of butter. The longer it is cooked the darker it gets. The darker it is the more flavor it adds.
Step 1: Assemble your Ingredients
for a nice thick stew use a 1:1:1 ratio
for every cup of liquid you want to thicken combine 1 tablespoon of butter with 1 tablespoon of flour
Step 2: Cook
Stir the flour in with the butter until well blended.
Once it is blended it can be used, or you can continue to cook the Roux to change how it affects the taste and thickness of your dish.
Continue to stir the Roux as it cooks.
I like to wait until the Roux achieves a rich peanut butter color before i consider it ready to use. This gives a nice balance between the flavor it adds to the dish and the thickness it provides.