Correct understanding the Four Foundations of Mindfulness of Buddhism is key to the realization of Enlightenment:

Bhikkhus, this is the direct path for the purification of beings, for the surmounting of sorrow and lamentation, for the disappearance of pain and grief, for the attainment of the true way, for the realization of Nibbāna – namely, the four foundations of mindfulness. 
~ M 10.2

The Four Foundations of Mindfulness is the practical application of the Not-self Doctrine.

The details of this practice, the Satipathana Sutta, can be found at the following link:  

Satipatthana Sutta: Satipatthana Sutta: Frames of Reference (accesstoinsight.org) 

Here I will bring to light insights that make this sutta comprehensible.   

The attention to the breathing and examination of the qualities of the body accomplish a disinterest in the body itself. But more important, it establishes of a point of view, or consciousness of the body as an object of consciousness.   

The key here is the emphasis on knowing. The student “knows” the breathing, “knows ‘I am breathing a long breath.'” Knowing is an activity of consciousness towards an object of consciousness. It is directed at, or intentional.  

The same awareness is expressed in the acts of reflection and contemplation: “he lives contemplating the body in the body. . . the monk reflects on this very body.”  

The practitioner contemplates “consciousness in consciousness.” Here the object of awareness is a state of consciousness:  

Herein, monks, a monk knows the consciousness with lust, as with lust; the consciousness without lust, as without lust; the consciousness with hate, as with hate; the consciousness without hate, as without hate. . . He lives contemplating origination factors in consciousness, or he lives contemplating dissolution-factors in consciousness. 

In general, the awareness is directed at the five aggregates (forms, feelings, perceptions, thoughts, and consciousness). The aim of distilling the existence of pure awareness as not the objects of awareness.  

The Not-self doctrine works to establish a point of view through understanding. The Four Foundations of Mindfulness reveals a practical method to know the mundane and unappealing nature of the five aggregates. This makes it possible for the mind to let go, rather than suppress the desire for the world.

To know, “the monk knows,” refers to an awareness and not to an understanding or thinking. This may not seem like much, but it is key to bringing about the awareness of awareness.  

Through the process of both knowing and understanding the transient nature of phenomena, the mind is freed.

But it is not just the establishing of a disinterest of phenomena. This would leave you with nothing. It is a process of realizing the “one who knows” as not being any phenomena. The “one who knows,” the meaning of the word Buddha, can only be thought about as a negation. That is the aim of the Not-self Doctrine.

Through meditation and mindfulness, the awareness of awareness is made real. The “one who knows” is known in knowing. It is a pure awareness that is revealed with Enlightenment.

Proper understanding the intent of the Four Foundations of Mindfulness avoids much confusion, and suffering, on the path to Enlightenment.