In his seminal teaching on the four foundations of mindfulness, the late Chögyam Trungpa Rinpoche explained how to practice mindfulness of mind.
Mindfulness of mind means being with one's mind. When you sit and meditate, you are there: you are being with your body, with your sense of life or survival, with your sense of effort, and at the same time, you are being with your mind. You are being there. Mindfulness of mind suggests a sense of presence and a sense of accuracy in terms of being there. You are there, therefore you can't miss yourself. If you are not there, then you might miss yourself. But that also would be a double take: if you realize you are not there, that means you are there. That brings you back to where you are—back to square one.
Really we operate on a very small basis. We think we are great, broadly significant, and that we cover a whole large area. We see ourselves as having a history and a future, and here we are in our big-deal present. But if we look at ourselves clearly in this very moment, we see we are just grains of sand—just little people concerned only with this little dot which is called newness.
We can only operate on one dot at a time, and mindfulness of mind approaches our experience in that way. We are there, and we approach ourselves on the very simple basis of that. That does not particularly have many dimensions, many perspectives; it is just a simple thing. Relating directly to this little dot of nowness is the right understanding of austerity. And if we work on this basis, it is possible to begin to see the truth of the matter, so to speak—to begin to see what nowness really means.
In sitting practice, or in the awareness practice of everyday life, for that matter, you are not trying to solve a wide array of problems. You are looking at one situation that is very limited. It is so limited that there is not even room to be claustrophobic. If it is not there, it is not there. You missed it. If it is there, it is there. That is the pinpoint of mindfulness of mind, that simplicity of total up-to-dateness, total directness. Mind functions singly. Once. And once. One thing at a time. The practice of mindfulness of mind is to be there with that one-shot perception, constantly. You get a complete picture from which nothing is missing: that is happening, now that is happening, now that is happening. There is no escape. Even if you focus yourself on escaping, that is also a one-shot movement of which you could be mindful. You can be mindful of your escape—of your sexual fantasy or your aggression fantasy.
Things always happen one at a time, in a direct, simple movement of mind. Therefore, in the technique of mindfulness of mind, it is traditionally recommended that you be aware of each single-shot perception of mind as thinking: "I am thinking I hear a sound." "I am thinking I smell a scent." "I am thinking I feel hot." "I am thinking I feel cold." Each one of these is a total approach to experience—very precise, very direct, one single movement of mind. Things always happen in that direct way.
Source:
http://www.shambhalasun.com/index.php?option=com_content&task=view&id=3389&Itemid=0