I put the macaroni (or I use tagliatelle or fettucine in) and cook it as per instruction - use a decent one but not necessarily a premium one - but not a cheap one. Then wash it with a little water (prevents sticking), drain and put in a buttered baking dish or casserole dish.
Make a white sauce (come back if you do not know how to do that - but essentially a white roux and slowly add/stir in to the flour/butter mix with a wooden spoon - it will quickly absorb early on in the heat of the pan. Make up enough (fresh) milk to cover the pasta.....at the end of the milk as it starts to get close to boiling and thickens up, I add some doublecream. Then you add grated cheese - cheddar suffices and usually 8 to 12 oz depending on quantities needed. I also like plenty of fresh ground black pepper in my sauce, sometimes some grain mustard as well. A touch of cayenne or chipotle pepper goes down well - and I also have used peppers for a Superbowl party in the USA
Cover with a good layer of breadcrumbs and some extra grated cheese. Some 1/4 or 1/2 tomatoes can be added on top for colour. (By the way for a variation, you can add sauteed leeks and bacon to the base mix). Bake for 20 to 30 mins at 200C - the crumbs may just need a very quick toasting at the end to add a little browning.