The white sauce recipe is here. The 'how to cook pasta' stuff is here. Combine them with a bit of grated strong cheese and - bingo! - you've got macaroni cheese. If you use about 400g of dried pasta, you'll be able to feed 4 or 5 hungry people. You can keep leftovers in the fridge. It's easy to reheat - just thin it with a little water as it's heating. It doesn't have to be macaroni. Try pasta twists or shells, even spaghetti will work. Here's a bit more detail on the relative quantities.
Ingredients
400g macaroni
800g - 1 litre of white sauce
150g of grated cheese
A quarter teaspoon (or less) of chilli or cayenne pepper
Salt
Cook the macaroni. It might take a little longer than the suggested
12 minutes, depending on how soft you like it. Macaroni is a thicker type of
pasta than most others (though it's not as thick as pasta shells - they take
ages to cook). Melt the cheese in the white sauce as it's bubbling at the end
of cooking time, stir in the pepper and salt to taste, then add the sauce to
the drained macaroni. Stir everything together and that's it ready. A green
salad and some sliced fresh tomatoes will go very well with this dish.