Now you make your first cut with your fillet knife. It should always be at an angle from just behind the head on the "back" side, meaning the side without the guts. Doing this gets you a little more meat. Take that cut down through to the tail. Be sure you are not cutting through the backbone. You want to cut over the backbone.
The way to do this is to slice down on this first cut, then turn the knife toward the fish's tail and slide it over the backbone.
Will you get it right every time? Not until you've done a few dozen fish. But keep at it.
A tip to remember: The meat at the edges of the fillet is not very useful. It is thin and its grain goes in another direction from the main fillet. This means you can run the knife out to the edges of the fillet, get a slightly raggedy edge and not have to freak out. We'll clean up the fillets later.