This is a rich, full-fat, baked dish.

One pound of macaroni for classic mac and cheese makes 5 to 8 servings. If it is a side dish, figure 8, but if it is the main dish, teens, or heavy eaters, go for 5-6. So the first decision you must make is, amounts.

A note about the cheese- if you have time, grate it yourself; cheaper for top quality cheese. Also, the texture of the sauce will be better with freshly grated cheese, because pre-grated cheese has a light coating to keep it from clumping. Get a tasty cheese for a top outcome.

Here are the basic proportions for classic baked macaroni and cheese, per pound of pasta:
1 pound macaroni (get a real, durum. semolina mac)
4 ounces (1/2 cup) butter
OPTIONAL 1/4-1/2 cup grated onion
1/2 cup flour
2 cups milk
2 cups cream
OR you can use 1 quart 1/2 and 1/2, or if you must, whole milk
2 teaspoons salt
1/8-1/4 teaspoon nutmeg (you can use pepper, but the nutmeg is better)
3/4 to 1 pound (4+ cups) cheddar cheese, tasty shredded sharp (this is the right cheese if it crumbles) OR combination to taste
OPTIONAL 1/2 cup shredded Parmesan (it steps up the cheesiness)
1 cup breadcrumbs, buttered, for topping OR Panko bread crumbs OR cracker crumbs (crispier)

For the cream sauce/ béchamel:

Heat the milk over medium-high heat until it just comes to a simmer, then turn off the heat and set aside.
In a large, heavy-bottomed pan, melt the butter over medium heat. Add the flour and whisk constantly until the mixture turns light brown in color, about 3 minutes. Remove from the heat. Note: IF IT SCORCHES, throw it away and start over! You can't rescue a scorched cream sauce.
While whisking constantly, slowly add the hot milk to the flour mixture until evenly combined and smooth. It will get very thick as you first add the milk, then thin out. Keep whisking.
Return the pan to medium-high heat and while whisking constantly, cook until the sauce thickens and coats the back of a spoon. Cooking 10 minutes gets rid of any floury taste. Stir in salt, taste, and add additional as desired. Remove from the heat and set aside.

Assembling

Butter or spray the baking pans and dust with grated Parmesan (old Kraft style).

Bring the water to a boil. 1 gallon takes about 20 minutes to boil. You need about 1 gallon for each 2 pounds, and you can only reuse the water once, so for a large amount of pasta, you need a LOT of pots.

Cook the pasta underdone, about 2-3 minutes short of aldente. This is important; it will cook further in the sauce, and can get mushy. Drain (if you are re-using the water, to save it, be careful).
Rinse immediately with cold water; set aside.
If you plan to bake the finished casserole right away, now is the time to turn on the oven. For temp see info below, and place the rack in the middle.
Place the reserved saucepan of béchamel over medium heat, warm it and stir in cheeses just until melted and smooth.

Now here is the decision point; if you have time for a full bake, just thoroughly mix in the pasta, transfer to the baking pans, put on the crumbs, bake 1 hour in the preheated oven at 350.