This is the most important part (aside from salting the pasta water). Good mac 'n' cheese doesn't come from shredding a bunch of cheese onto cooked pasta. You're going to fold that pasta into a silky cheesy sauce.
1. Melt butter (like 4 Tbsp. or 1/2 a stick)
2. Add an equal amount of flour (so again, 4 Tbsp.)
3. Whisk to combine and cook for a few minutes so that flour doesn't taste raw. Now you've got what is called a "roux" — equal parts butter and flour cooked together.
4. Add around 2.5 cups of milk or cream and whisk again. Watch it thicken and magically turn into sauce.
And that, sirs, is how to make bechamel sauce, one of the five mother sauces of classic French cuisine. Technically it needs some grated nutmeg and white pepper to be a bechamel, but Jacques Pepin isn't going to bust in and bitch slap you for skipping that part, should you choose to. (Although don't you kind of wish he would?)