Following the flow of thought without intervening
Sit with your eyes closed in a chair, an armchair or assume your favourite meditation position; since it is a mental exercise, posture does not really matter; the important thing is that it is comfortable enough not to cause distraction. If you are sleepy or tired, postpone the practice until a more convenient time, sprinkle your face with cold water or take a few deep breaths to regain attention.
Start observing the flow of thoughts. In the usual waking activity, thoughts and emotional states are intertwined; the sight of a person or an object can generate feelings and thoughts of love, hate, desire and so on; in turn, an idea causes a chain of thoughts that are difficult to escape. The purpose of this practice is to inhibit this reaction by simply observing the arising of thoughts in a relaxed way as if you were an external observer and without forcing their elimination.
Obviously, that observer is only a metaphor to contextualise the principle of inaction, which is the basis of the exercise. Thinking of being an inert observer is in itself a thought, which risks generating an infinite recursion of observers who observe the observer. Thought is a manifestation of being, so there is no absolute separation between the thinker and the thought. As noted above, effective action is to break the emotional attachment with the source thought, not generate other thoughts; the same is true for disturbing emotions or thoughts that arise unexpectedly, such as when we hear sudden noises during the practice session. Again, it is not the sound that creates an obstacle to meditation but our reaction to it.
With practice, you will notice a thinning of thoughts that, without their emotional nourishment, will tend to disappear spontaneously as they arise. Start with a 2–3-minute session in the morning, repeated in the evening, increasing by one minute each day. When you manage to maintain a conscious detachment from thoughts for ten minutes, the exercise is over, and you can move on to the next step.