Easy. You set the water to boil on the stove. While that's going you start the roux, which is just butter and flour. That takes about two minutes to cook, then you can add your milk and salt and turn down the heat. Add your pasta to the water. Now add your cheese to the sauce. I know how much I need so I just cut off a hunk and crumble it up into pieces into the pot. Once the cheese has melted, your pasta should be done so drain it and stir it into the pot with the cheese. You are done, enjoy your mac and cheese.
The box stuff is easier because it's less steps, but doesn't take any less time to prepare.