If it is a large piece, use a fine saw to cut groves about every 1/8 inch or so from the thick side to the point. Then use a flat blade screw driver to gently break the tabs between the cuts. Finally, use a sanding block to finish the surface. 180 grit or finer should do.
If it's a small piece, use a hard backed sanding block or file to cut the taper.
Go slow either way.