I have been making this as a basis for my gumbo for many years. It is really easy to make if you have patience. To me, it is not much different than making gravy as this is also how I make my gravy.
Take equal amount of an oil and flour, setting the flour aside until your oil, grease or lard is melted in your frying pan or stock pot. For a roux, butter is not recommended as it is prone to burning. Maybe, 3 Tablespoons of each to a cup of liquid, would be a good starting point to learn. If making this for a recipe, use the recipes guidelines.
I also recommend using
cast iron as it is easy to keep a low constant temperature. I also like to make mine with bacon grease.
Slowly add the flour to your melted lard or oil, stirring constantly, then cook the flour and fat over medium heat until they form a smooth paste while continuing to stir the mixture.
Next, lower the heat and cook until the roux is a dark, chocolate brown. Stir very frequently and scrape the bottom of the pot to avoid burning. This
should take an hour or more. I don't cook mine that long though the longer you cook it the more flavor it will have.
Now add your chopped vegetables to brown, like onions, celery and bell pepper or what your recipe calls for.
At this point you are ready to add liquids or broth, stirring while adding the liquid slowly.
Here is an oven method that I have not tried.
Oven: While not the traditional way of making brown roux, we feel that the oven method is easier and more fool-proof, plus you don’t have to baby-sit the roux quite so closely.
Preheat the oven to 375°. In a dutch oven or other oven-safe dish, start the roux on the stove-top by cooking the roux over medium heat until it forms a paste. Move the dish to the oven, uncovered, and cook for ten minutes. Stir the roux, being sure to scrape the bottom of the pan. Continue cooking and stirring every 10 minutes until the roux is a dark, chocolate brown. This should take an hour or more.
https://www.thekitchn.com/tip-how-to-make-a-brown-roux-73211 That link also has a recipe and instructions for a Roux on the stove
Here are a couple of videos that might help. Everyone, including me, has their own techniques: