There is no 'bad" Zazen, even the bad Zazen. So, we sit looking at the clear, open blue sky without clouds (clouds represent thoughts, the blue sky represents the mind clear of thoughts). Clouds drift in and out, while some days are very blue and some days completely cloudy without any blue sky visible. It is just the weather, which changes. The blue sky is there even when hidden by the clouds. Also, nothing "wrong" with clouds ... it is all the natural sky. Something like that. Blue sky and clouds are all the sky, do not wish to break up the sky. The sky and the weather are just the sky and the weather on any given day.
We do not try to "silence the thoughts before they arise" in Skikantaza. It is more that we allow the thoughts that naturally drift into mind to to naturally drift out of mind, much as clouds naturally drift in and out of a clear blue sky. In this way, return again and again to the open, clear blue sky.
One of the key points about Master Dogen's approach to Zazen is to allow the clouds (of thought) to drift naturally out of mind (our thoughts of this and that, likes and dislikes, judgments, events, etc) and we come back again and again to the clear blue sky. Do that again and again, 100 billion times and 100 billion times again. So, no need to "catch" the thoughts and chase them away, even as we seek during Zazen to find the open, blue sky.
HOWEVER, Dogen taught "non-thinking" (also called "thinking not thinking"). That means that there is nothing "wrong" with the clouds. It is not that blue sky is "good" while clouds are "bad" (some Buddhism and other Eastern philosophies imply that). We allow the clouds to drift out of mind, but neither do we resent the clouds when present or on very cloudy days. Even on those days when the sky is all cloudy, and not an inch of blue is present, the blue sky is still there behind the clouds. WE DO NOT SEEK TO BREAK UP OR RESIST ANY PART OF THE SKY, CLOUDS OR BLUE ... It is all the unbroken sky. Understand?
Thus, allowing things to just be the way they are, no judging, not resisting, being with the flow, allowing 'happy' days to be happy and 'sad' days to be sad, all while dropping all idea of 'happy' and 'sad', whether really enjoying or really not enjoying ... fully dropping away any and all thought of doing Zazen 'right' or doing it 'wrong' ... THIS IS DOING IT RIGHT. And when you are doing it right, it will usually feel like you are doing it right, for there is no resistance, and a great sense of balance.
Fighting things, wishing things were some other way that how they are, judging, resisting, going against the grain and the flow, wishing 'sad' days were happy or 'happy' days were happier ... filled with a sense of self bumping up against all the other 'selfs', with a mind held by thoughts of doing Zazen 'right' or doing it 'wrong' ... THIS IS DOING ZAZEN WRONG. And when you are doing it wrong, it will usually feel like you are doing it wrong, for there is resistance, and a sense of imbalance.
But as well, even at those times when Zazen feels 'wrong', when there is resistance or imbalance ... it is still 'right', still 'Zazen', still just what it is. IT CANNOT BE WRONG. This last point is vital to understanding.
Yes, that is a Koan. Is it clear? Please really really penetrate in your body and mind what I just wrote.