Wash it down under the cold tap to remove any sand.
Don't bother gutting it.
Locate the spine and cut down to it.
Pressing the knife against the bones cut towards the edge in the direction of the bones. Folding the flesh up as you do so makes life easier.
Repeat for the other side of the spine.
Turn over and remove the flesh on the ventral side in the same way.
You should end up with four fillets.
Skin the fish by holding the fish tailend skin side down. Keep the knife pressed downwards against the skin and movements short, this takes practice.
Better still, Gut the fish stuff with bread crumbs, egg and onions and grill whole.