Best way to sharpen them is to send them to Ron at KME and pay whatever he wants.
Forget about a file, diamond is the only way to go. If you are going to do it yourself, you need MAJOR time and patience. A proper low speed, water cooled, power sharpener,with a jig would be the ticket. I made a jig to hand sharpen (posted this last year)on diamond plates which works well but time consuming. Many people give up. I did not. Copy the Ashby advise.
Some have used a power sander (like a belt sander) but if you heat/overheat them, they are ruined.Without power, consider 2 hrs a piece required without the KME jig.
IMHO
Rob