Watch James Martin fillet mackerel and improve your knife skills using a boning knife and a filleting knife. Start with the head end, insert the knife just behind the gills, and cut along the backbone and against the spine, all the way down towards the tail. The fillet will come straight off. Do the same with the other side. You will be left with the carcass (head and spine). Discard.
This method works with other round fish, leaving you with two fillets.