Cut the head off, slit the abdomen and remove the gut. Leave the black lining, as it later indicates that all the bones have been removed.
Lay the Garfish on its back and cut along the right side (your right side - the Garfish's left) of the exposed backbone at the base of the ribs. The cut should be through the ribs and as deep as the dorsal spines on the backbone. The completed fillet looks better if the knife does not go deep enough to cut the skin.
Continue to cut through to the tail.
Lay the fish out flat. Now work the knife under the dorsal spine and the ribs.
Lay your forefinger along the ribs, pressing firmly to keep them flat, and you can feel the knife slicing just under the bones.
The knife slides better if it is frequently dipped in water. Carry the cut through to the tail.
Lay the knife flat under the remaining ribs and, using a slicing motion, remove the bones.
Discard the head, guts and bones. You are now left with a reasonable Garfish fillet.