I carry many things with me. You can never be too prepared.
*eventually produces a long, folded reed, still green. he carefully cuts it in half, handing one piece to Mitsuru*
This should do. For the grass you choose, the wider the better. It should be at least a handswidth long. Then, take it between your thumbs like so, and pull it taut, like this. *demonstrates--it's a bit clumsy with the metal thumb and all, but you wonder if he's practiced beforehand*