I sharpen mine using a large flat stone and a strap of leather glued to a pice of wood.
Start by pulling them (FLAT) across the stone from back to tip.
Start with 15 reps on all 3 sides, then 13, 11, 9, and so on till you get to one and see how sharp they are, repeat the process if nessary.
Once they are sharp repeat the process on the leather strap, it takes the burs off and the cutting edge will be polished.
If they are real dull or have tool marks in them, dont be afraid to use a fine metal file on them to start with. just be sure to stroke the same amount of times on each side.
If you have any questions PM me and I will give you my phone #