By ‘watching’ thoughts, the meaning is that you shall just acknowledge and accept that these thoughts are there without thinking into the merits of the content of these thoughts and then bring back the attention to focus on the present activity (breathing or any other). It is not possible to forcibly control the thought from coming but it is certainly possible to not get involved in these in any way. There is no need to “think over” these thoughts, just accept whatever these are and do not make any judgments. You have to let these pass-by and see these going off, just as a person sees vehicles passing by him.