Initial positioning for blowing the first notes
There are three postural components to put in place when blowing the first sounds:
1) Good upright vertical body, neck and head alignment
1) The best postures for blowing are either kneeling in seiza position or standing. Whichever position you take, ensure that you are truly vertical all the way up to the top of your head. A mirror can help with this positioning. A common mistake is to allow the head to fall forward from the neck, or to round the shoulders.
2) The back of the shakuhachi has to be positioned so that the utaguchi blowing edge situates itself just above the bottom lip, by about 2mm or less. This necessitates sliding the back of the flute up and down the chin carefully until that position is found. The flute does not necessarily fit best in the natural cavity under the bottom lip. Also, don't wave it about in the air as you try to blow in it. Anchor it softly under your lower lip and hold it in place there by your two hands on the flute.
3) The partner to position 2 is the angle of the flute....this angle helps to create the right gap between the bottom lip and the utaguchi edge. For many players, 45 degrees is fine, but small angle adjustments may have to be made for the best positioning.
Once these three postural aspects are in place, they can remain as they are. They don't need to vary as you play. From this point onwards, it is the lips (in particular the top lip) that does the necessary work for embouchure shaping.
How to blow shakuhahchi
A good utilitarian embouchure often has two properties: the top lip sits slightly forward of the bottom lip and the aperture between the lips is a flattened narrow slot.
By using a trifold mirror, like a bathroom mirror, you can observe these two parameters and adjust accordingly.
Begin by blowing gently across the utaguchi cut edge. There is a point at which you'll find a 'sweet spot' where the sound suddenly emerges strongly and clearly. Visualize the flute itself pulling the tone out of your mouth like soft caramel.
Not much effort is required to create a small flattened nozzle between the lips. Blowing too hard is often the first error a new player will make.
The tongue is quite relaxed, floating in the lower basin of the mouth cavity, but the end of the tongue anchors itself on the gum line below the bottom front teeth. There are variations on this anchor point.
Limit the amount of movement you go through (with flute and lips) to find a sound. Shakuhachi tonal and pitch modulation takes place in a physical micro arena. Stillness (not rigidity) is necessary to maintain a clear strong tone. Very tiny lip and chin movements can result in a big effect on the quality of the tone.
By flicking out the tongue to touch the utaguchi edge every time you settle into a blowing position, you can get to know the best geography of the lip/flute relationship. The tongue is a tester, to see if you are in the right position with the flute.
Contrary to intuition, one does not need a lot of breath to play shakuhachi. Some older and very famous players have only one lung, but they still can play with full power and control.
Breathing begins and ends at the 'hara', the abdominal muscle wall area around the belly button. Breathe in by distending the abdominal wall gently outwards and avoiding a 'shoulder shrug' at the end of the inhalation.
The exhalation of the breathe through the flute can be visualized as a soft wind sourced deep inside your lower abdomen. All the throat, mouth and lung cavities should be open and free of muscle tension, as the air leaves the body.
Spend a good amount of time just blowing long tones, each one a complete slow exhalation after a deep inhalation. Don't stab at it with short breaths. Concentrate on maintaining an even pitch as your breath diminishes. One way to do this is to slightly raise your chin (kari) as the sound begins to fade in strength . Thus you can maintain pitch as the breath runs out.
The second octave
The second octave requires an increase in air pressure in the bore of the flute. The interesting thing about this is that one needs almost no effort to make that happen. A combination of a slightly flatter lip opening and a slightly closer positioning of the lips to the utaguchi cut edge, will speed the air up sufficiently to kick the sound into the second octave. This is SUBTLE. No efforting is needed! If you watch a player do this, you will hardly notice any difference in blowing and embouchure from lower octave to upper octave. The better and stronger your lower octave sound is, the better and easier to access your upper octave sound will be.
If you find yourself making a huge effort in order to blow the second octave, catch yourself, and remember success comes with embouchure shaping, not so much breath pressure. Try to flatten the slot between your lips by bringing the top lip down closer to the bottom lip. That is the key!
Going tighter and with a smaller lip aperture will produce a 'purer, harder' sound. Loosen the embouchure and soften the lips and the sound becomes 'looser', with more breathy overtones. An experienced player will be able to create a solid, perceptible tonal 'core' in either of these cases, around which play beautiful harmonics. There is incredible subtlety in embouchure technique. You only have to ask a cross-section of good players to blow on your own flute to appreciate how personal sound production is. The great mystery is: what happens to the embouchure shape over the years of a relationship with shakuhachi, that produces mature sound...a sound that never stops developing, and which increases in sophistication, seemingly without end? This is largely player-specific, not flute-specific.
If you can, find a teacher
Whatever your ultimate intention with shakuhachi, you will need a palette of possibilities, otherwise you may meander around the same small pond of note progressions and dynamics forever. The most startling experience you can have as a new student is to have your teacher play your own flute.
Most classical Japanese music can only come to a student via transmission from a teacher. There is so much subtlety involved in the exposition of honkyoku zen solo pieces, for example, they cannot be notated successfully, only demonstrated.