Cut off the fins with a pair of scissors and remove the head by cutting just behind the gills. Slit the fish along the belly and remove the insides with kitchen paper. (At certain times of the year the fish may have roe [egg sacs] inside which may be cooked and eaten too.)
Open the belly out and stand the fish on a board, skin side up. Press down firmly with the fingers along the centre back of the fish.
Turn the fish over and ease the backbone away from the flesh. Cut off at the tail. Remove any small, loose bones. Rinse the fish using cold water and dry on kitchen paper.