Cook the pasta according to the package directions.
As it cooks, toss the cheeses with the flour to combine them well.
Heat the milk in a medium-large saucepan and when it starts to steam, add the cheese mixture in handfuls, stirring to melt each handful before adding the next.
Drain the pasta and add to the cheese sauce; stir to coat everything well. If you like, keep it on low heat for a few minutes, stirring often, to thicken. It can also be transferred to a baking dish, topped with more grated cheese or buttered bread crumbs, and baked at 350˚F for 20 minutes, or until bubbly and golden.