I have had requests for a beveled edge red tortoise pick guard for a Martin that did not come with one. My problem is that in making the beveled edge I always end up with rather deep scratches that are impossible to rub out.
The last one I did I started with 320 grit silicon carbide paper. After getting the bevel so therewas no line where it meets the flt surface of the pick guard I used gradually higher grit silicon carbide paper with water. Then, a buffing wheel with Tripoli followed by a buffing compound for restoring gloss to plastics. I get a nice high gloss but on close inspection there are a lot of scratches. If I were to start with a higher # grit it would take an unreasonable length of time to make the bevel.
I made one by using an Exacto knife or razor blade.r first but that made it impossible to not have a line where the bevel meets the flat surface.
I know Martin has a Delmare pickguard available at a much higher price, but it does not come with the beveled edge. So I would still have the same problem with it.
The current Martin tortoise guards are pretty awful: halftone dots in colors not quite found in nature. They remind me of a bad hairpiece. I've replaced quite a few of those Martin guards (and black ones, and...) with real celluloid ones. I get the celluloid from Axiom, and it comes with clean surfaces. I lightly doublestick-tape the roughed out guard to a plywood backing and do the final trim there, wood and all. Then I hand-bevel the edge much as Greg describes, being careful not to booger the main surface. When I have the chamfer nice and round, I buff with a nice big machine buff. No sanding scratches to get out, and the plywood block supports the edge so it doesn't distort or burn.
You can keep scraping round and round, finessing a curve until there's no ridge between the edge and the main surface. If the scraper is really sharp, there are no lines or irregularities to deal with—the buffer is the next stop. I basically never use abrasive papers on guards or flatpicks. From beginning end, it takes me about half an hour to lay out a guard and have it buffed and ready for the double-sided adhesive film.