Make a roux - equal amounts melted butter and flour (I use 4 tbsp of each, use 2 each for a smaller batch.) - and cook for a few minutes to get the starchiness out of the flour but do not let it burn or darken. Then add half the number of Tbsp in cups of milk - i.e. if you used 4 Tbsp, add 2 cups of milk. Using a whisk, cook over medium heat, stirring constantly, until the mixture comes to a boil and thickens. Season with S & P or a seasoning salt.
Now, start adding bags of grated cheese, or cheese you have grated up yourself. I like to add several different varieties. Sharp cheddar is nice, mixed with some mozzarella, and some colby-jack. A little smoky cheese mixes nicely in there too. If you really want it to be rich and creamy melt some cream cheese in there too. A little bit of Dijon mustard mixed in adds an interesting tang too.
Now just boil up your macaroni, drain it, and mix it into the sauce. That's it, you don't bake it or anything. Its super creamy and delicious. After making it a few times you can experiment with the mix of cheeses to make it just how you want it to taste.