How to sharpen chalk?

If you meant sharpen so here is a link.

https://www.google.com/search?q=chalk+sharpener&rlz=1C1BLWB_enUS700US700&source=lnms&tbm=isch&sa=X&ved=0ahUKEwiY1eztv-3TAhWJ34MKHTUzA5AQ_AUICygC&biw=1024&bih=714

First thought, rub it on sandpaper!

Try a cheap pencil sharpener with a large hole. Of course you won't sharpen to a point, but a few turns should shape it for writing.

I use sandpaper. You have to sharpen often, though, as chalk is so soft.

I use sandpaper when I need to sharpen chalk. . You can buy strips sandpaper stapled to small wooden slat. or you can make your own go to the hardware store and buy a sheet of medium fine sandpaper. Cut the sandpaper in to one inch wide and for to six inches long, staple one end of the stacks of sandpaper to stiff cardboard cut to the same size as the sandpaper strips.

If it is the skinny chalk I have used a pencil sharpener and gotten good results. Sandpaper works best on sidewalk chalk size.

Use sandpaper.

You can also sharpen it on unsealed concrete.

Try an emery board or nail file