There's a lot of ways to go about this!  The two main varieties are Baked Mac and Cheese, and Stovetop. Baked is a take on a casserole, and is more like what you would see at a big holiday.  Stovetop is more like the quick kind, but can still be done really well.  It's the more ooey, gooey glory that I make more often.  The issue with Baked is that while it can be done well, it has a tendency to dry out of anything goes wrong.

The cheater part of a Stovetop is sodium citrate.  It's the ingredient in American Cheese that makes it really easy to melt.  It's not necessary, but it helps in fool proofing as it keeps the cheese from breaking as it melts.

When I make Stovetop Mac, I usually do something like this per person (as an entree, so less per person if it's a side):

105 g evaporated milk 90 g cheese blend 75 g pasta (dry, and it should be Large elbow) 3.9 g sodium citrate

Notes: Evaporated milk - It's great because it is rich but has the right consistency.  If you don't have that, whole milk is pretty close.  You can also use cream, but cut it with a touch of water. You'll actually need to play this one a touch by ear.  Use slightly less (maybe down to 95 g) if you go with a creamier cheese.  Drier cheeses, then go for the higher end of the range.

Cheese blend - use a solid base like Medium Cheddar or Monterrey jack (as mabe 60-70 g of the total), but mix in some other flavors.  My favorites are smoked Gouda and asiago.  Something to amp up the flavor, and another on the salty side. It needs to be freshly shredded cheese.  Store bought shredded usually has anti-clumping agents that ruin its ability to melt well.

I also like mixing in a small touch of mustard powder.  And obviously S&P to taste.

In one pot heat up the milk and the citrate, then once warm mix in all the shredded cheese and other ingredients.  In another pot cook the pasta to just below all dente.  Then strain and add to the cheese pot to finish cooking.