How to flake rock sea salt?
Flakes form when salty liquid evaporates. With naturally formed flake salt, like harvested sea salt flakes, it's skimmed off the top of seawater pools. I have read, but not confirmed, that higher-volume commercial flake salt manufacturers boil salt water dry and collect the salt from the cooking vessels.

So the good news is that you don't need to buy any fancy gadgets; the bad news is that if you're just interested in obtaining flaky salt and not doing this for some other reason, you probably don't want to do this. 

You could try dissolving your salt in water and then boiling it down, but I'd be surprised if you managed to get a consistent flake scraping it off the bottom of the pan. Pretty sure that just buying flake salt and saving your big old salt nugs for grinding or grating would be the way to go... but hey, it's your salt.