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This is, of course, the proper way of doing things.

My practical version for everyday cooking/trying not to consume large amounts of cream looks like -

Bring a couple cups of milk (usually have 2% in the house) to a near simmer, then whisk in another cup of milk with a few fat spoonfuls of flour mixed into it (as a slurry.) Let it barely come to a boil for a few minutes, thickening the milk mixture, scraping the bottom and sides slowly the whole time. Once it coats the back of the spoon well turn the heat way down and add salt, pepper, garlic powder, onion powder to taste. Turn the heat off and add an obscene amount of shredded cheese (usually whatever partially used bags and random slices that need to be used from the fridge.) Once melted, double check seasoning and add to fully cooked pasta for a stovetop version. For baked Mac and cheese, undercook the pasta a bit and save some shredded cheese for the top.

Boiling the mix after the cheese has been added increases the risk of the cheese splitting and getting oily. Adding a slice of American cheese provides enough sodium citrate to create the same smooth, saucy texture. Shredding your own cheese may also be best practice, but I've never had an issue with pre shredded cheese.