Meditation is a skill that everybody can learn, and it can be practised while sitting, walking, lying down or standing. The Japanese word Zen comes from Sanskrit (dhyana) and means meditation. There are three elements to pay attention to: posture, breathing and the state of mind. The first two determine the third; conversely, the state of mind influences physical posture and breathing.
Posture: Often the greatest emphasis is placed on sitting meditation – zazen. Traditionally practised cross-legged; in all of them the alignment of the spine is crucial. The spine should be long and follow its natural curvature. Knees touching the ground and the buttocks form a stable tripod-type support. One of the best description of zazen posture is in ‘Zen training’ by Katsuki Sekida. There are also presentations on the web. However the face-to-face instructions by an experienced teacher or practitioner are hard to beat.
The above positions allow practitioner for sitting still for long periods of time. However they may not be practicable for Westerners. Then sitting on a chair is recommended, provided that the chair has the right height so that feet rest on the ground, and it is not sloping backwards. The same principle (as sitting cross-legged) of supporting the spine applies. The tripod of stability is achieved by separating knees, placing feet flat on the ground and firmly sitting on the sitting bones.
In all positions the bottom part of the body should be rock-solid, and the upper part ‘light as a feather’.
Breathing: through the nose. It should be deep, abdominal, not enforced. During a meditation period the breaths would naturally lengthen, and become lighter. For some practitioners it may take a while to change from shallow breathing (top of the lungs) into deep breathing. Typically, the physiological effect would be relaxation and feeling of calm.
The meditation techniques of counting the breath or following the breath are used as vehicles for developing awareness and concentration.
The state of mind: This is best experienced rather than described. Various stages can be achieved through sustained practice. The key skill is focusing the mind so that distractions can be dealt with, and the mind guided back to its original task. This skill serves the practitioner to be ‘here and now’, to face sitting with one’s self, and to enter deep meditation where there is no longer an awareness of body or mind.
Further reading: http://plato.stanford.edu/entries/japanese-zen/