I preheat my oven to 350°F before I even touch the stove. Then I bring a big pot of salted water to a boil and cook the macaroni until it's almost tender but not quite there yet. I drain it well in a colander and set it aside while I work on the sauce.
I melt the butter in a microwave-safe bowl or a saucepan on the stove. Once it's fully melted, I whisk in the flour and whole milk until the mixture turns smooth. A pinch of salt and pepper goes in here too, right before the cheese joins the party.
I drop the cubed Velveeta straight into that milk mixture and microwave it in 30-second bursts, stirring after each one. It takes a few rounds, but the cheese eventually turns into one silky, golden sauce with zero lumps left behind.
I fold the drained macaroni right into the cheese sauce until every noodle is coated. Then I spread it into a greased 9x13 dish and bake it uncovered at 350°F for about 20 minutes, until it's bubbling and just barely golden on top.
I always let the dish sit for about 5 minutes once it comes out of the oven. This short rest lets the sauce set up a little, so it holds together beautifully when you scoop it instead of running all over the plate.