Place the flounder on a cutting board. Use a knife to scrape from the tail to the head to remove scales and slime. Flip the fish and repeat. Rinse under running water and pat dry.

Turn the fish belly-up. Make a small incision near the gills and remove the internal organs. Rinse thoroughly to clean scales and blood, then pat dry.

Place the fish with the back facing up and the head to the left. Make diagonal cuts near the pectoral and pelvic fins down to the backbone. Repeat near the tail. Make cuts along the gills and a straight incision down the center. Slice along the backbone to remove one fillet. Flip the fish and repeat for the other side.