Start by placing a 10-12 single cut file flat on a workbench. File tang away from you, and then point the head down the middle of the file, using the original bevel of the blade, push into the file with a little downward pressure. Rotate to the off side and repeat 2-3 strokes on each side. Watch for a telltale burr. You should remove the burr, by stropping on a leather strap.
If you use a file or stone and try to sharpen it like a knife, from the side, it causes the file to rock over the blade, rounding and dulling. It has to be absolutely flat on the file.
If the burr doesn' come off, use a stone, repeating the push into the stone for a few strokes should do it.
At this point if you have done it correctly it should be sharp. Next, use a 6-8" file and drag the corner of the file from the back of the head to the front, in a motion to the outside. This creates a small cerrated edge. Be sure to drag the corner of the file from the head to the outside, not into the blade. Two or three times and check the edge. Strope the head on leather, and repeat if needed.
This method produces a very sharp, cerrated blade, with very light cerrations, with only a file. I carry a file in the field and can achieve good results. Let me know how it works for you.