For the fillings, cut the tuna with the grain into 1cm/½in wide strips. Cut the cucumber into 1cm/½in thick strips.
Place the makisu on the work surface, then place a nori sheet on it horizontally, rough-side up. Spread a quarter of the su-meshi over the nori to cover evenly, leaving a 1cm/½in margin on the side furthest from you. Press firmly to smooth