Sand paper on a piece of broomhandle will do.
Some have said an empty paper towel roll too.
Smith's & EZE Lap make oval diamond sharpeners that would be ideal.
Mine is still sharp after all the chopping I did with it last year & only needs touching up on a round ceramic rod.
I have a set of EZE Lap flat paddle type diamond sharpeners that work fine.
They are only 3/4" wide.
Grits from extra fine 1200- extra coarse 150.