Usage GRIND <item> <EDGE\POINT\WEIGHT\BALANCE\POWER>.
This will only work with a working grindstone. To use one, you must turn it to get it up to a high rate of speed, and then push grindstone with <item>. This will prepare the <item> for grinding, but you will need to repeat as needed.
Be aware that as you increase one category others will change also. Edge indicates how good an edge you want. Point indicates how sharp of a tip. Balance is how well it fits in your hand for parrying. Weight is, well, how heavy it is to wield. Power is how well you are able to apply its attributes.