Melt the lard or heat up the vegetable oil over high heat in a large cast iron skillet.
White: Gradually whisk in the flour until it is well mixed, then turn the heat down to medium and whisk constantly. After 2-5 minutes, you'll start to see a change in color. This stage is called a white roux.
Blonde: Keep whisking and after 5-10 minutes you'll see the next color change: a blonde roux. If you like, exchange your whisk for a wooden spoon, but keep stirring constantly so the flour doesn't burn.
Light Peanut Butter: After stirring for 15-30 minutes, the flour will darken to a golden brown or peanut butter color. For this stage, keep the heat to be anywhere between medium low and medium.
Copper Penny: This will require about 30-45 minutes of constant stirring. Be patient--it's worth it. The smell of the flour browning should give off a nutty aroma. If you smell it burning, lower the heat or move the cast iron skillet off the burner for a while. If there are black flecks in the roux, it is burnt and you'll have to start over, as a burnt roux will taste bitter.
Milk Chocolate: After 45-60 minutes of stirring, you'll achieve a milk chocolate color roux, which is perfect for a Cajun gumbo.
Dark Chocolate: After you reached the milk chocolate roux stage, remove the cast iron skillet from the heat and continue stirring for a couple of minutes. Allow the roux to cool in the skillet, and it will darken because of residual heat. Now, you'll have a wonderful base for your gumbo.