Maguro is the reddest part of the tuna, which is near the spine. Generally, lighter and shinier meat is the best quality. It is easy to tell when maguro is old, as the meat gets darker red and leathery.
Cut the chunk of tuna you buy, seperating the reddest part. Cut this part into long rectangular fillets, 1 inch thick. Then, cut diagonally into 1/8 inch (5mm) slices, slanting the knife to the right and starting from the left end.
One end of the fllet will be triangular, in which case you should make a slit starting from the wide end and open it out into a triangle.