When you sit, do not sit in order to become something. Do not sit to become calm. Do not sit to become a good Zen student. Do not even sit to become enlightened. If you sit for any of these reasons, you are not really sitting — you are striving, with a meditation costume on.
Just sit. This is harder than it sounds.
What you do
Take your posture. Spine straight as a young pine. Hands in your lap, left palm resting on right palm, thumbs barely touching, making a soft oval. Eyes open, lowered, looking at nothing in particular about a meter in front of you on the floor. Mouth closed, tongue lightly on the roof of the mouth.
Breathe through the nose. Do not count, do not control — though counting from one to ten on the exhale is a fine gate to walk through when you are starting. When you reach ten, begin again at one. When you lose count, begin again at one. The losing and beginning again is not the failure of the practice. It is the practice.
When thoughts come
Thoughts will come. Let them. Do not chase them, do not fight them. Picture clouds passing across the open sky — the sky does not grasp them, does not push them away, and is not damaged by them. You are the sky. The thoughts are weather.
If you notice you have been carried away on a thought, smile a little inside yourself. Return. Spine straight. Breath. One. Two.
What you might miss
Beginners often ask: how will I know if I am doing it correctly? You will not. There is no certificate, no glow, no feeling that arrives to confirm you have done well. Some days the sit will feel deep. Some days you will sit for thirty minutes and the entire time will be about your knee. Both are zazen. Sit anyway.
Do not collect special experiences. If a beautiful stillness comes, let it come. If it leaves, let it leave. Do not chase it back. The next sit is a fresh sit.