The sharp edge has to be straight, not curved like most kitchen knives, otherwise it will not cut properly.
It should only be sharpened on one side, with any resulting burr taken off the flat side, same as a chisel or plane blade.
It will never be razor sharp unless it is high quality steel.
Felco the secateur people do a nice little diamond sharpening stone for about £14. Not in most g-centres, so try internet. Else use trad stone, which is better for getting a straight edge initially.
Do not use under arms unless you intend replacing a limb!