Dry Roux

Select a container for storing your roux in the freezer in, fill it to the top, with AP flour, add 2Tbs more, then spread evenly on a large baking sheet.
Bake at 350 stirring throughly every 10 minutes until Roux is lightly browned, about the color of peanut butter.
Ideally, you'd share the oven with something else so that you're not running the oven for just the roux.
Baking roux takes around 30 to 40 minutes.

The darker the roux, the less thickening power it has.