I do
2 cups uncooked pasta (then cook it - retaining 2 cups of the pasta water at the end)

Heat/melt 3 tablespoons butter and 3 tablespoons flour. Add slowly one cup milk, and then the 2 cups pasta water.

Then add approx 3/4 cup grated cheese and salt and pepper to taste then the cooked pasta.

You can eat it like this, but i much prefer to bake it in the oven sprinkled with breadcrumbs and a little grated cheese.