Heat fat in a skillet. Use a cast-iron skillet or another heavy frying pan. Place the pan over medium high heat and add the fat. Let the fat heat until it has just melted. If using oil, let oil heat up for 2 minutes.

Add the flour. Place the flour in the skillet with the fat. Use a whisk to begin stirring the mixture constantly. Use the whisk to break up any lumps in the flour and distribute it evenly through the melted fat.

For a thicker roux: add an extra 1/4 cup of flour.

For a thinner roux: reduce the flour by 1/4 cup.

Cook the roux. Continue whisking the roux constantly while it cooks. The mixture will begin to thin and darken after several minutes. Continue cooking until it has the color and consistency you need.

Remove the roux from heat. When the roux has reached the color and texture you want, remove it from the heat. It's now ready to use.

Store the roux. If you want to use the roux later, place it in a food storage container and keep it covered in the refrigerator. You can scoop out as much as you need on a recipe by recipe basis.