Once it's melted and just sizzling, add about a spoonful of white flour, and stir until you have a smooth paste.
Cook the roux until it's slightly darker to give the sauce a richer flavour.
Add a spoonful of milk and stir until smooth. This is the critical step, but it works out fine unless you let it burn.
Keep adding milk, a spoonful or two at a time, and mixing, until the sauce is quite liquid and you're not having to stir much.
Add the rest of the milk a little faster, still stiring.
Cook until the sauce is heated through, and bubbling.
Add unhealthy amounts of your prefered cheese, grated, and keep stiring.
Cook until the sauce starts to thicken and remove from the heat. It'll thicken more as it cools.
It seems like a lot of steps, but once you get the hang of it, it's quick and reliable. The recipies that add flour to a lot if liquid have a tendency to get lumpy, this one cooks the flour in fat first and adds liquid slowly, which pretty much eliminates the issue.
