Add oil to a heavy bottomed skillet and apply medium heat.
Add the flour to the oil and stir together until totally combined, using a roux spoon (a flat edged spatula or a whisk is also fine).
Continue with the heat and watch the skillet closely. Stir continuously.
If you feel the heat is too high turn it down a little. The color should change slowly.
You’ll pass a blonde roux (often used for things like Alfredo sauces, or Mac and cheese etc), then have a peanut butter colored roux, good for soups etc. You’re looking to get the roux to a color of Hershey Chocolate. Very dark (but not burnt).
Be careful after you turn off the heat, as the skillet is still hot and will continue to cook the roux. Continue stirring until it has cooled a bit, unless you continue with a recipe, then add the other ingredients.