Lay the fish on a board with the head pointing away from you. Cut around the head and down the centre or lateral line of the fish right through to the backbone
Working on the fillet nearest to you, insert the point of the knife under the flesh at the head end. Keeping the knife blade parallel to the bones, slice away the fillet using long sweeping strokes. Remove the other fillet in the same way but turn the fish round so that the tail is pointing away from you and cut from tail to head.
Repeat the whole process on the other side to obtain the remaining two fillets, giving you four quarter-cut fillets. (Sometimes only one fillet is taken from each side and these are called cross-cut fillets.)