I have a 3"x 6" diamond hone. I sit it flush against the angled cutting wings and make 2-3 light passes. Then I set the mill on the hone with all cutting surfaces in contact then I slowly turn it clockwise for several turns. Cuts beautifully.
I use a small file also...one thing I would add that makes it easier for me is to take something like a black marker and color in the complete area of the 'wings'...this way you know if/when you have hit the entire wing sharpened.
I use a small square ceramic file. It is the perfect size to fit the pen mill wings. As long as you make a couple of passes with it as soon as you detect dullness, it keeps them nice and sharp.