I recently figured them out myself, by using a chainsaw file and a "wheelie sharpener" combo. My process is to use the chainsaw file, just like you would a flat file on a straight edge broadhead. I make numerous "hard" strokes changing the angle of the edge to make it a little flatter. Once I have both sides worked down pretty good, I lighten up on the pressure some, but not really to a "final edge type softness."
At this point I run the heads through a "wheelie sharpener". Lots of people say they can get a really good edge with just the wheelie sharpener, and that's why I purchased it with the heads, but I never could get them sharp enough to cut hair. After changing the blade angle, it only takes a few passes through the "wheelie sharpener" to easily get them hair shaving sharp.