Add 1/4 Cup flour to cast iron skillet and whisk over medium heat. The flour should combine with the oil to make a roux. Make sure to scrape up any bits of flour stuck to the pan. The roux should be a thick paste. If it's not sticking together, you need more oil, so add another tablespoon. If it's really runny, you need more flour. Cook roux until it starts to brown slightly, about 2 minutes.