Our view of ourselves and the world we live in is a distorted view of the Reality; instead of experiencing the undivided, Pure-Consciousness-Bliss 'Self' everywhere and in everyone, we experience separateness and dead matter! This is Superimposition, and it is caused due to our ignorance of our real nature. After repeated experiences of this illusory existence, in many lives, the soul understands the futility of it and then moves on the path of renunciation, practices four-fold spiritual disciplines (as shown in the diagram below); and thus becomes fit to inquire into the nature of unchanging Self. Such a soul receives the grace of God/Self and through it comes to a Guru who dispels the darkness of ignorance and shows the light of the Self. The way pointed out by the compassionate Guru marks the process of De-Superimposition.

Some souls - through practice of self-discipline, concentration, austerities etc. in many lives - have purified their psyche so thoroughly that when they meet Self-Realized Guru, the Guru has only to utter the words 'Thou Art That' (तत्वमसि Tatvamasi) to them, and they immediately experience the Truth in the form: 'I Am the [Infinite] Brahman' (अहं ब्रह्मास्मि Ahambrahmasmi). Such souls and their meeting with such a Guru is extremely rare phenomenon; though not impossible. Such aspirants are categorized as having Highest Competency (उत्तम अधिकारी Uttam Adhikari).

The second category of aspirants are those who, after hearing the Truth from Guru (श्रवण Shravana), have to cogitate on what is heard (मनन Manana), and then have to continuously meditate on it (निदिध्यासन Nididhyasan) for long time to realize the Truth. They are categorized as having Medium Competency (मध्यम अधिकारी Madhyam Adhikari).

Hearing about Truth (श्रवण) is not just listening It from Guru or reading about It, but it comprises determining through six characteristic signs (start & end of subject matter, it's originality, eulogy, repetition, result, demonstration with references) that the entire Vedanta philosophy proclaims the Absolute, non-dual Brahman. According the Vedanta texts, the statement 'Thou Art That' is to be properly understood because a human being in his/her 'limited- phenomenal' form can't be one with the infinite Brahman. So what does the Great Dictum (महावाक्य Mahavakya) - 'Thou Art That' - mean?
Scriptures talk about following three types of relational identities that are to be properly understood with regard to 'Thou Art That':

a) सामानाधिकरण्य Samanadhikaranya - Relation between two words having same substratum; the words 'Thou' and 'That' have the same substratum of Pure Consciousness.

b) विशेषण-विशेष्य-भाव Visheshan-Visheshya-Bhava - Relation due to exclusion of contradictory ideas; the idea of immediacy conveyed by 'Thou' and the idea of remoteness conveyed by 'That', being contradictory to each other are excluded by their combined usage.

c) लक्ष्य-लक्षण Lakshya-Lakshana - Relation due to removing the contradictory part and retaining the common implied part; the idea of immediacy conveyed by 'Thou' and the idea of remoteness conveyed by 'That' are to be excluded and common substratum of Pure Consciousness, which is implied, is to be retained.

After thus hearing the Truth, the aspirant has to cogitate (मनन) on the Truth within his/her mind - with arguments supporting it and removing doubts against it. The next step is unceasing meditation (निदिध्यासन) on the Truth without being obstructed by any other foreign idea. This leads to 'self-conscious Absorption' (सविकल्प समाधी Savikalpa Samadhi) i.e. the aspirant's mind attains almost oneness with the infinite Pure Consciousness but does not fully merge into It; the slight trace of the triad - knower, known, and knowledge - remains. In this state, the awareness of relative phenomenon and Absolute Reality persists simultaneously; Reality is experienced in and through the names and forms of the phenomenon.

Next and the final state is 'Absorption without self-consciousness' (निर्विकल्प समाधी Nirvikalpa Samadhi), in this state the triad of knower, known, and knowledge dissolves in Reality. It is like salt merging in water without leaving behind any trace; thus the mind completely merges in the Self. Swami Vivekananda explains the steps of Shravan, Manan, and Nididhyasan as:

‘Brahman is without action, Atman is Brahman, and we are Atman; knowledge like this takes off all error. It must be heard, apprehended intellectually, and lastly realised. Cogitating is applying reason and establishing this knowledge in ourselves by reason. Realising is making it a part of our lives by constant thinking of it. This constant thought or Dhyana is as oil that pours in one unbroken line from vessel to vessel; Dhyana rolls the mind in this thought day and night and so helps us to attain to liberation.’

Nirvikalpa Samadhi is the highest goal for a human being; it is a state in which the apparently unceasing transmigration of soul through various bodies stops finally and fully. This is Liberation (मुक्ति Mukti).

A point that aspirants need to note is the difference between the state of Samadhi and deep-sleep which have external similarities as in both of them the duality (of subject and object) is not cognized. But while in Samadhi the mind takes the form of infinite Pure Consciousness, in deep-sleep it plunges into ignorance. Also the effects of these two states are different on the aspirant: after Samadhi, the aspirant is enlightened and is supremely endowed with wisdom while after deep-sleep, he/she remains the same ignorant person.

The aspirant of Lowest Competency (अधम अधिकारी Adham Adhikari) requires help of rituals, books, temples, images, self-less work (Karma-Yoga) etc. for a long time before he/she can develop the strength to do unceasing meditation (निदिध्यासन) on the Self. Unfortunately majority of aspirants are in this lowest category and hence so much confusion about Religion/Spirituality!

Obstacles to Self-Realization

The state in which majority of humanity finds itself is a superimposed state, a confused state of delusion in which the nature of oneself and world can not be properly determined. Vedanta scriptures tell us that we are in this superimposed state due to repeated habits/thought-patterns that we have developed for ourselves over many life-times. Naturally for de-superimposition and to achieve Samadhi, we have to reverse the trend; the task seems as difficult as reversing the direction of flow of a river! Upanishads rightly describe the path of de-superimposition as walking on razor's edge since spiritual practice consists of rejecting everything that we think or perceive as non-Self and maintaining the awareness of 'Witness Self' (of which we have very vague idea or intuition!). The main obstacles to Samadhi mentioned in scriptures are: torpidity, distraction, attachment and enjoyment of higher bliss.

Torpidity (लय Laya) is the lapse of mind in sleep as it is unable to focus on the Absolute; this is most widely experienced by spiritual aspirants or meditators during their attempts to dive deep into the mind.
Distraction (विक्षेप Vikshep) is the mind's tendency to run-away towards external objects and stray thoughts instead of focusing of the Self; mind has learned to flow towards external objects and hence it finds turning inward extremely difficult.
Attachment (कषाय Kashaya) is the numbness of mind due to its attachment to sense-pleasures and desires; this bluntness of mind prevents it from focusing on the Self.
Enjoyment of higher bliss (रसास्वादन Rasaswadan) results when mind is able to achieve Savikalpa Samadhi (Absorption with self-consciousness); but this prevents the mind from rising higher towards Nirvikalpa Samadhi.

After prolonged practice, austerity, concentration etc. the spiritual aspirant achieves the blessed state of Nirvikalpa Samadhi by the Grace of Guru/God. Thus the process of De-Superimposition is completed and the aspirant is firmly established in the Self; his/her cycle of transmigration is ceased and supreme contentment is established.

(For further study of the topic kindly refer to books published by Ramakrishna Math and Ramakrishna Mission, Chinmaya Mission, and others on Upanishads, Bhagawad-Gita, works of Shankaracharya like Vivekachudamani, Tattwa-bodha, Aparokshanubhuti, Drg-Drsya-Viveka, Atmabodha etc. and Vedanta treatise like Vedanta-Sara of Sadananda.)