Understanding the Four Foundations of Mindfulness is key to benefiting from this essential teaching of Buddhism: 

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 (Bhikkhu Ñāṇamoli and Bhikkhu Bodhi trans.) 

The Four Foundations of Mindfulness is a method for the realization of pure awareness, or the original mind, as not the objects of consciousness. The thorough details of this practice, the Satipathana Sutta, can be found at the following link:  

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

A lighter version of the sutta is discussed by Bhikkhu Bodhi at the following link:  

The Buddha’s Four Foundations of Mindfulness | Lion’s Roar (lionsroar.com) 

Here I will bring to light some insights that make the sutta more comprehensible and practical.   

First, it is evident that it is derived from the Not-self Doctrine.

The attention to the breathing and the examination qualities of the physical body accomplishes a disinterest in the body itself, and more importantly, the establishing of a point of view, or distance from the consciousness of the body as an object of consciousness.   

The key here is to note that the meditator “knows” the breathing, “knows ‘I am breathing in a long breath’.” Knowing is an activity of consciousness towards an object of consciousness: it is directed at, or intentional.  

The same awareness as directed at something is indicated by the acts of reflection and contemplation: “he lives contemplating the body in the body. . . the monk reflects on this very body.”  

More importantly, and more illuminating, is that the meditator contemplates consciousness in consciousness. Here the object of awareness, in consciousness, 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. 

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

While the Not-self doctrine works to establish the point of view, or distance, from the objects of consciousness, it does not clearly disclose these objects as undesirable, only as impermanent.  

The sutta of the Four Foundations of Mindfulness reveals the mundane and unappealing nature of the five aggregates through the comprehension and focused attention of mindfulness.  

It is necessary to view the objects of consciousness as not-self, unsatisfactory, and a cause of suffering for the mind to release its identification and desire for them. In other words, to not become them and get lost in the world of things.  

To know, as in for example “the monk knows, ‘The enlightenment-factor of mindfulness is in me’,” refers to pure awareness and not to an understanding or comprehending. It is not thinking about it but just knowing that it exists as such. This may not seem like a difference, but it is what brings about the manifestation of pure awareness.  

What purifies the mind is not the rejection of forms, feelings, perceptions, mental formations, or the consciousness of these but the knowing these as Not-self. Note that there is no concept of “I” or self in the knowing.

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