The top side of a flounder is much thicker with meat than the underside, but the underside is worth filleting, too. There’s not necessarily any trick to doing it—it’s the same as filleting most other body types of fish. But cutting the thin side of a flounder trips up some people.
FS member arunram prepared a helpful video on how he fillets a flounder. It’s instructive, full of good tips and straight to the point. Check it out here.