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To paraphrase my master, the formal practice of Zazen is as follows:

If a Dojin is entering a Zendo to meditate:
Bow lightly with your hands in Gassho (palms together, fingers up), and
step in with the left foot first (the left side represents compassion).  The
path you take to your seat should be just a foot or so in front of the row
of cushions.  Any turn you make should be made squarely in the corners
instead of in a sweeping radius. Your pace should be respectfully metered,
deliberate, & mindful but otherwise normal.

If at any time you should pass in front of the altar, you should
respectfully bow Gassho.  Walk to your seat, stand facing the bench or
cushion, and bow Gassho.  After this, turn clockwise and bow to the
Sangha (group) before sitting.

At the designated time, the Han (a wooden plank drum) will be struck in a
prescribed manner in three rounds as a call to formal meditation.
Immediately afterwards, a bell is struck three times, signaling the
beginning of Zazen. By the time the Han is on the second roll down, you
should be seated and in correct posture. When the bell is struck for the
third time, you should be completely "adjusted" so that you have no
further need to move throughout the course of the meditation period.
The normal sitting period is a minimum of thirty minutes. You should be
settled into your meditative posture well before the bell has been struck
for the third time.  This position should be one of the recommended
postures that you can maintain without moving for a prolonged period, with
your hands held in the Hokkaijoin Mudra (sometimes called the "cosmic
mudra").  

If you must make slight adjustments to ease pain in muscles or joints, do
so slowly and quietly. If you must cough or sneeze, cover your mouth and
resume posture without saying anything.  Avoid any unnecessary actions
like scratching that can be overcome by determination.  If you must move
because the position you have chosen has become unbearably painful, then
Gassho, re-adjust yourself, Gassho again and resume Zazen.  Do this
quietly and carefully to keep from disturbing others.

When the time comes to leave your seat, bow to show respect and
gratitude for your meditation, then rise and turn clockwise to face your
seat.  If you are using a cushion, fluff it back into shape and brush away
any hair or dust.  Bow to your bench or cushion, turn clockwise to face
into the room, and bow to the Sangha.

Note: If you have any problems with the recommended Zendo etiquette
and sitting postures please adjust accordingly.  What is important is that
you are comfortable with your participation and meditative posture.  Being
quiet and mindful should be your foremost considerations.
ZENDO ETIQUETTE