The main points of mindfulness practice are as follows:
First, the object of mindfulness is oneself. It is important to be mindful of the phenomena that are occurring inside of oneself. Those things that occur outside of oneself do not have a great deal of meaning because deliverance·Nibbāna is something one achieves from within.
The Mahāsatipaṭṭāna Discourse (D22), among others, said to divide oneself into body, feeling, mind, and mental objects and then further subdivide these into 14, nine, 16, and five types, respectively, or 44 objects in total. Then, select only one of these objects. Of course, he said, one could select an object of mindfulness from another’s body, feeling, mind, and mental objects (D22, etc.) using the same method. However, the starting point is always oneself.
Second, most important is deconstruction of conceptual [paññatti] entities. The Greater Discourse on the Foundations of Mindfulness (D22) points out the Buddha’s reason for deconstructing the objects of mindfulness into body, feeling, mind, and mental objects. The dhamma with the general characteristics of the impermanence·suffering·non-self will appear clearly when conceptual entities such as ‘I’, mine, mountain, river, computer, automobile, the universe, etc. are deconstructed. Once the impermanence·suffering·non-self of the dhamma are seen, craving and ignorance for those entities should not arise.
Deconstruction is important. The being of ‘I’ is at the core of deconstruction. Sentient beings presume a true-self and then want to grab onto the unchanging true-self. This is the greatest clinging among all clingings. If something is not deconstructed, one gets fooled by conceptual entities. When one deconstructs and sees the dhamma, then one realizes deliverance·Nibbāna, here and now.
Third, the object is important to mindfulness. The Buddha’s suggestion in the Greater Discourse on the Foundations of Mindfulness [the Mahāsatipaṭṭāna Discourse] to deconstruct ‘I’ into body, feeling, mind, and mental objects and to further deconstruct these into 21 or 44 types of mindfulness objects is significant.
If one pursues the question ‘Who or what am I?’ while ‘I’ is left as a conceptual being, then one can fall into a false notion of the true self, the genuine self, and the great self. Which will lead to inversions of perception as the self being permanent, pleasurable, existing, and attractive [常樂我淨]. However, when ‘I’ is deconstructed as body, feeling, mind, and mental objects and is seen with insight, impermanence, suffering, non-self, and unattractiveness [無常, 苦, 無我, 不淨] would be deeply felt. As a result, one will realize the revulsion-dispassion-deliverance-knowledge of liberation. [Introduction to Early Buddhism, P89]
Fourth, samatha and vipassanā are unified by mindfulness. The practice of Buddhism is largely classified into samatha and vipassanā. Samatha practice is synonymous with the concentration practice and has been translated as止 [di] in Chinese. Vipassanā practice is synonymous with the insight practice and has been translated as觀 [kwan] in Chinese. Samatha-vipassanā [di-kwan] is the practice that has sustained Chinese Buddhism.
Without mindfulness, neither concentration nor insight practices are possible. Samatha takes a sign, a conceptual [paññatti] entity, as an object. Vipassanā takes dhamma that arises and perishes moment-to-moment as an object. Regardless of the object, without mindfulness there cannot be samatha that concentrates on a sign nor vipassanā that sees clearly into the impermanence·suffering·non-self of dhamma.
Mindfulness, very important mental formations, is common to both kinds of practices. Therefore, it is emphasized in the Fire Discourse, “But mindfulness, bhikkhus, I say is always useful.” (S46:53)
Fifth, it is concluded in the Connected Discourses on the Foundations of Mindfulness [the Satipatthāna-Saṁyutta Discourse] and the Greater Discourse on the Foundations of Mindfulness [the Mahāsatipaṭṭhāna Discourse] that one must obtain the highest knowledge [aññā] by observing the Four Noble Truths. Among the three characteristics [impermanence·suffering·non-self] of all conditioned phenomena, by clearly knowing the characteristic of the suffering, the origin of suffering, the cessation of suffering, and the way leading to the cessation of suffering, one can realize deliverance·Nibbāna. According to the Path of Purification, there are three gateways to liberation. These are the impermanence·suffering·non-self.
“When one who has great resolution brings [formations] to mind as impermanent, one acquires the signless liberation. When one who has great tranquility brings [them] to mind as suffering, he acquires the desireless liberation. When one who has great wisdom brings [them] to mind as non-self, he acquires the void liberation.” (Vis.XXI.70)
Therefore, the practice of mindfulness concludes in the desireless liberation through insight of suffering. According to Early Buddhist discourses, thoroughly observing the Four Noble Truths is the way to obtain the liberation and the realization of Nibbāna.
“Bhikkhus, these four foundations of mindfulness, when developed and cultivated, lead to utter revulsion, to dispassion, to cessation, to peace, to direct knowledge, to enlightenment, to Nibbāna.” (S47:32, the Dispassion Discourse)
“The object of mindfulness is ‘I.’ Being mindful of phenomena unfolding within ‘I’ is important because deliverance·Nibbāna is something ‘I’ achieve from within.”
[Introduction to Early Buddhism authored by Bhikkhu Kakmuk and translated by Nancy Acord]
 Samatha is concentration meditation practice.
 Vipassanā is insight meditation practice.