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, insight and brilliance..
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, cloudiness, greyness.
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.
I hope that helps. And by the way, as we "do" Zazen", Zazen does us ... and ultimately there is just the "doing" ... .