After mastering the basics of meditation, the final challenge is "just sitting"
not focusing on the breath or body or chanting mantras or letting the mind wander
At the site of your regular sitting, spread out thick matting and place a cushion above it. Sit either in the full-lotus or half- lotus position. In the full-lotus position you first place your right foot on your left thigh and your left foot on your right thigh. In the half-lotus you simply press your left foot against your right thigh. You should have your robes and belt loosely bound and arranged in order. Then place your right hand on your left leg and your left palm (facing upwards) on your right palm, thumb tips touching. Thus sit upright in correct bodily posture, neither inclining to the left nor to the right, neither leaning forward nor backward. Be sure your ears are on a plane with your shoulders and your nose in line with your navel. Place your tongue against the front roof of your mouth, with teeth and lips both shut. Your eyes should always remain open, and you should breathe gently through your nose.
Once you have adjusted your posture, take a deep breath, inhale and exhale, rock your body right and left and settle into a steady, immobile sitting position. Think of not thinking. How do you think of not thinking? Non-thinking in itself is the essential art of zazen.
The zazen I speak of is not learning meditation. It is simply the Dharma-gate of repose and bliss, the practice-realization of totally culminated enlightenment. It is the manifestation of ultimate reality. Traps and snares can never reach it. Once its heart is grasped, you are like the tiger when he enters the mountain. For you must know that just there (in zazen) the right Dharma is manifesting itself and that from the first, dullness and distraction are struck aside.
When you arise from sitting, move slowly and quietly, calmly and deliberately. Do not rise suddenly or abruptly. In surveying the past, we find that transcendence of both unenlightenment and enlightenment, and dying while either sitting or standing, have all depended entirely on the strength [of zazen].