First thing you need to do is clean the fish, NO NEED to scale the fish, the salt will take care of that, just make a slit on the belly of the fish, or side, and remove entrails and wash out with clean water, remove the gills if leaving head on, if you wish to remove the head, no problem, do that now.
For the lemon grass, lime leaves, and ginger; if you buy the fish from an open fresh market, many times these will be included with the price of the fish, they are tied together and simply added to the bag with your fish. If you are buying the fish in say, Tesco, then you need these items on hand.
Stuff the cavity with the lemon grass, lime leaves, and ginger. Place in a large bowl.
Now for the salt, regular table salt is fine, if you can locate sea salt, that is fine, the salt is not for a flavoring of the meat, it is to remove the skin, salt is cheap, let the salt do the work, hence the reason not to scale the fish.
With the bag of salt, generously pour that over the fish in the bowl, you want to cover the fish in salt, spread it around with your hands as well, turn the fish over and repeat on the other side, you cannot have too much, you want the fish to be white.
With a hot grill, place the fish on the grill and cook about 8-10 minutes or so, then flip the fish over and cook the other side. It will get charred, that is the salt charring.
Take fish off from grill and place on a plate, at the table, using 2 forks, simply peel off the skin off and enjoy.