Mix in a microwave-safe bowl equal parts flour and oil and wisk with fork until smooth (no lumps). Do not use plastic. This stuff will get very hot. Microwave on high. Stop frequently to stir to check on the color of the roux. Do not grab this bowl without gloves, it will light you up!
I don't know why the color on top of the cooking flour is always lighter brown than the roux below the surface. Also, the roux will continue to cook for a little while after you have shut the microwave off. So, you will have to stop the cooking a little shy of the final color of brown you are aiming for. The cooking roux always starts to brown much quicker in the very center of the bowl. So, remember to stop and stir to mix the flour and to check on the color beneath the surface. I guess you could add a little room temperature oil to help stop the browning.
If you notice any black flecks, discard the whole mixture and start over. I have made this sound way more difficult than it actually is. The goal is to brown the flour until you have the darkness you like. I make large batches at a time and freeze it. My wife simply scoops out however much she wants when preparing a meal and re-freezes the rest.