An alternative method follow the white sauce method thought I think this is a simpler variation.
Might be an idea to grease a milk saucepan or a larger saucepan.
Pour in the milk and set on a low heat. Gradually add the grated cheese stirring often with a wooden spoon. At no point allow the milk come to boil.
You will still need to stir this milk cheese mixture often even while doing the next section.
After adding all the cheese to the milk put the flour in a jug or other container and stirring add a few dessert spoons of the milk with added cheese. When smooth add some more of the milk/cheese liquid stirring together. Now stir back into the saucepan. You can bring the liquid to a high heat but not I stress to boiing. Season now with the salt & pepper and maybe other herbs of your choice like parsely for example.
With this mixture pour over for example prepared macaronico in an ovenproof dish.