How the Shakuhachi is actually used as a meditation practice (suizen 吹禅, "blowing Zen") is a topic often overlooked and seldom referred to explicitly. I have selected some of the teachings that I have received over the years from my teachers. Those hints might help you to get started if you already are a Shakuhachi player.
1) Play regularly.
Needless to say that you need to play as much as you can. "I'm not in the mood today" etc. is a silly excuse, Shakuhachi can be played in any mood, that's the point.
2) Play by heart.
You need to memorize some pieces to be able to practice suizen. Start with short ones. Shakuhachi is about experiencing sound, not about looking at scores.
3) Play freely.
Don't get caught in the form of a piece too much. Play the way you remember it and check afterwards where you need to revise.
4) Halt and watch.
From time to time you should stop in the midway of a piece and listen/watch the quality of the silence (shikan止観). Some pieces have little hints in the notation where to stop, most don't. After playing for a while your breath will change and so will your ability to watch your mind.
5) Finish your practice without finishing your piece.
When you finish your practice session always stop in the midway of a piece. The unfinished piece will resonate through your day and your everyday life will naturally become part of your meditation.