It is really quite easy, but I can't stress enough how important it is to be patient and double check your measurements.
I do them in my sleep now....or maybe I should be sleeping and instead i'm working but anyway, be meticulous with the measurements.
My belly to back measurements on the tennon (male portion) are incredibly close to parallel (with in .003"). Side to side will taper ever so slightly, usually not more than .010 -.015.
The "REAL" important detail is to make sure all edges are straight and not concaved (hour glass shaped).
One thing I have done differently that has made life a whole lot easier was to make a solid sanding block that is as wide or wider than the tennon. I made mine out of a huge block of phenolic and it is 2" wide. I put two sided tape on it and change the paper freguently. This allows me to keep all surface square. With a file (narrower than the tennon) there is a unavoidable tendancy to file unevenly leaving you with concave sides on your tennon.
best of luck and if you need advice, feel free to give me a call.
thanks, bigjim