1.) Go to hardware store and but a 9" x 12" scrap of 1/2" plate glass. It doesn't have to be tempered glass.
2.) Go to auto parts store and purchase one sheet each of the following wet/dry sandpaper grits: 600, 1000, & 2000 (or whatever the highest is they have). If you have a Woodcraft store in your city, they will also have this same kind of sandpaper in grits well above 2,000.
3.) When you get home, cut your sheets of sandpaper in half or thirds the long way. That way you get more mileage out of each sheet.
4.) Starting with the 600 grit, wet it completely with water then spread it flat on the plate glass (grit side up). The water should help "stick" it to the glass. If it starts to move on you, just dump some more water on the sandpaper. The water becomes both your paper adhesive and your sharpening lubricant.
5.) Take a sharpie marker and black out all edges that need to be sharpened. Just the edge itself, not the whole blade.
6.) Begin sharpening the broadhead just like you would on a diamond plate or flat stone. Flip it to the next side when all of the black magic marker is gone from the side you've been working with.
7.) Once you've made it all the way around, move up to the next higher grit of sandpaper and repeat steps 4 - 6.
8.) Continue moving up in grits until you've made it through all grits, repeating steps 4 - 6 with each grit change.
9.) Now, if you have a leather strop, go ahead and lightly strop each side just to remove any micro burr that may exist.