Removing the blood line from the cavity gets rid of any bitterness.
Lift up the gill flap behind the cheek of the head and, using kitchen scissors, remove the dark, frilly gills. Repeat on the other side of the fish.
If any scales remain, hold the tail and, using the back of a knife, scrape the skin at a slight angle, working towards the head. Trim the fins. Cut across the tail to shorten it by half, then cut a V shape into the tail.
Wash the salmon under cold water and open it on the belly side where the fishmonger has slit it. Remove the blood line lying along the backbone using a spoon. Rinse and wipe inside and out with paper towels.