It is kind of tough to get it perfectly flat without the right tools.
The method I use is to remove the pin and chuck it up in your dremel or drill press, so the tip is just barely over the surface of the tool, just above flush with the tip of the chuck. Spin it lightly against the face of a dremel cutting wheel to get it nice and flat. Then while still in the chuck polish it up lightly with increasing grits of sandpaper.
Read this tutorial. It is invaluable to follow or develop a method of your own....
