Always cut any meat and fish against the grain! You'll want to use an extremely sharp knife. Slice the tuna fillet against the grain, and at a 25 degree angle. You'll want your slices to be anywhere from 1/8, 1/4, and no larger then 1/2 inch thick pieces. Do not saw at the fish with a knife. Your knife should be sharp enough to just put pressure on the fish, and drag your knife in one, long stroke.