Start with a gutted and scaled fish, a long flexible knife and a damp cloth under the board so it does not slide.
Cut behind the head down to the bone, turn the knife flat and run it along the backbone to the tail with long strokes. Turn the fish over and do the same on the other side.
Trim the belly bones, feel for pin bones with your fingertips and pull them out with tweezers. Or bring the fish to the counter and we do it while you watch.