To fillet the halibut, you make a cut down the length of the spine, then across the front of the fillets. Make sure you don't cut through and into the stomach: That fillet will be shorter. The rest is really pretty simple. As you peel back the fillet from the spine, make long, smooth cuts, flexing the knife blade down onto the ribs to get all the meat until you get to the skin at the bottom of the fillet. If the fish is large enough, you can take the cheeks out by inserting the knife at the back of the muscle on the gill plate, cutting underneath it to flip it over, then peeling it off the hard plate under the skin.