Mentally "note" what is happening, a simple label such as "walking", "thinking" etc.
Keep the mental label light and transparent. Be careful not to make the note so loud that it drowns out the actual experience.
Remember it is simply a pointer or a reminder and not meant to substitute for the experience itself.
Think of putting 95% of your attention on whatever you are experiencing and 5% on the labeling of it.
With thinking, it is helpful to distinguish thoughts about the past and future with the labels "remembering" and "planning," respectively. Such labels are very useful in freeing us from being lost in the content of our experience and instead coming to a greater understanding of it as a process.
In formal practice, the most common notes are "in," "out" or "rising," "falling" for the sitting meditation, depending upon where you feel the breath most clearly, and "left," "right" or "lifting," "moving," "placing" for the walking meditation, depending upon the speed of your walking.
Drop in a mental note occasionally as a reminder during periods of non-formal practice, for example, while eating.
Keep the timing of the note correct, not too soon and not too late.
Extend the noting to routine activities usually performed out of habit, where you seldom bring awareness to your actions. Examples might be walking to the bathroom, putting on or washing your clothes, bathing, or brushing your teeth. Extend the noting to include intentions and the resultant movement of the body before dramatic changes in body posture.
Mental notes such as "intending," "reaching," "touching," or "standing," can help ground our awareness in our bodies and extend the sense of wakefulness beyond the boundaries of formal sitting and walking periods.
The mental note of "seeing" can be particularly helpful in counteracting the tendency of our attention to rush out through the eyes and get lost in the external world.
The mental note of "hearing" can likewise keep us from getting lost in habitual reactions to sounds.
Mental noting reveals what our experience is, how we relate to it, and what happens to it when we become aware of it.
It helps us to see the basic characteristics of impermanence, unreliability, and lack of independent existence that all things share.
Do things change when we pay close attention to them? How do they change? What is our relationship to these changing experiences?
These questions lead to a fundamental shift in our understanding of freedom. Remember that mental noting is simply a tool. Know when to use it and when to let it go.
Noting is unnecessary when mindfulness is strong and continuous or when the objects being attended to are too numerous or rapid in appearance to note precisely. In that case, a general note (such as "thinking," instead of trying to note the specific kinds of thoughts) can be helpful.
It is always possible, if mindfulness is strong enough, to be fully aware of an experience even though you may not be able to note it.
When noting is used properly, it leads to the realization of no-self (anatta), the crown jewel of the Buddha's teaching, as we come to understand directly that there are experiences but no-one to whom they refer.
As the Buddha said in the Bahiya Sutta, "Just this is the end of suffering."