Zazen is to sit firm, upright, comfortable and still, while ceasing the pursuit for thoughts and sense perceptions. In zazen, we let all thoughts, feelings and images come and go, just as they are, without any analysis or judgement. We just sit. This act of just sitting is called shikantaza in Japanese.
We sit like a heavy rock on the bottom of a river, letting all waves and streams flow us by. They will calm down by themselves. We let go of all struggles and goals, and we sit with everything as it is, here and now. Zazen is not about achieving something different. Rather, it is to return to a natural balance, clarity and presence.
A period of zazen is usually about 20-30 minutes long, but it can be longer or shorter. Zazen is never some kind of competition. If we sit several periods of zazen, we usually do kinhin in between. Kinhin is like zazen, but we walk slowly instead of sitting.