Before going on to specific instructions, let us examine the reason behind it. We are, after all, trying to develop awareness. In fact there are meditations of that nature. They are sometimes referred to as unstructured meditation and they are quite difficult. The mind is tricky.
Thought is an inherently complicated procedure.
By that we mean we become trapped, wrapped up, and stuck in the thought chain. One thought leads to another which leads to another, and another, and another, and so on. Fifteen minutes later we suddenly wake up and realize we spent that whole time stuck in a daydream or sexual fantasy or a set of worries about our bills or whatever. There is a difference between being aware of a thought and thinking a thought. That difference is very subtle.