When you are practicing zazen, do not try to stop your thinking. Let it stop by itself. It something comes into your mind, let it come in, and let it go out. It will not stay long. When you try to stop your thinking, it means you are bothered by it. Do not be bothered by anything. It appears as if something comes from outside your mind, but actually it is only the waves of your mind, and if you are not bothered by the waves, gradually they will become calmer and calmer. In five or at most ten minutes, your mind will be completely serene and calm. At that time your breathing will become quite slow, while your pulse will become a little faster.
It will take quite a long time before you find your calm, serene mind in your practice. Many sensations come, many thoughts or images arise, but they are just waves of your own mind. Nothing comes from outside your mind. Usually we think of our mind as receiving impressions and experiences from outside, but that is not a true understanding of our mind. The true understanding is that the mind includes everything; when you think something comes from outside it means only that something appears in your mind. Nothing outside yourself can cause any trouble. You yourself make the waves in your mind. If you leave your mind as it is, it will become calm. This mind is called big mind.