First, cook the pasta according to package instructions. After cooking you can put in butter in the pasta while hot (tastes better but i won't recommend it if you're on a diet). Next, just heat the milk until it thickens a little (but do not burn!!! i've had experience with that and you won't like the taste believe me). Add in the tuna flakes, salt and pepper to taste and you're ready to go. It's real quick to make, like 15 to 30 minutes or so.
Note: You can substitue an all-purpose cream for the evaporated milk.