Now you place the fish down on the board. Why is this its own step? Because alignment matters. Astute observers of the picture will notice that I am left-handed because you always place the fish's head where you can anchor it with your "off," or non-cutting hand.
Now keep in mind there are many ways to fillet a flatfish; I am showing the one I have used to fillet hundreds (if not thousands) of Atlantic winter flounder and summer fluke over the years. Many people will start from the tail end -- if I were to do that, I'd flip the fish around. Either way, you need to anchor the fish with your off hand.