To marinate the mackerel, place fillets on a flat plate, sprinkle salt on both sides and rub in gently. Leave for 1 hour and then rinse under cold running water and pat dry with a paper towel. Put fillets into a deeper dish, cover with the rice vinegar and leave for 45 minutes. Remove mackerel and pat dry.
Run your finger along the fillet and remove any bones with tweezers. Then slice the mackerel into pieces as long as a nori sheet and 1cm wide.