I have always heard people saying that scriptural or bookish knowledge of the Self is not sufficient and that one needs to realize the Self within. My question is, how does one realize the Self within? ... Do advaitins meditate on Nirguna Nirakara Brahman? If so, how is it possible to meditate on something which has no form?
In Understanding Advaitha book 1 by Amrut, two types of meditation are prescribed for Advaithans. From Page 135 of the book -
Two types of Sadhana can be practiced by Advaita Vedantin
1 ) Pratik Upasana : OM
Just like one can find the ocean shore by moving in the direction of sound of waves, one can go into the source of OM, which is Brahman / Atman. After that even OM rests. There is only you and nothing else. This is Advaita Sthiti called as Nirvikalp Samadhi. But again mind rises, as it was sleeping or temporarily subdued, and one is again in duality. The process repeats until everything that is dumped into the mind is uprooted by OM. After that there is mano-nasa (destruction of mind). This is Sahaj Samadhi. After this state, when one opens eyes, he / she sees that the world which we thoughts as an illusion, is nothing but Brahman itself (sarvam khaldalam Brahman everything 'Else' is (also) Brahman ). Meditation is over. One is Jivan Mukta and has achieved moksha. Practically, initially, when one is taught - I am not this not this, my family is not mine, my friends are not mine, nothing is mine, etc, one feels that suddenly the joy, enjoyment, ananda, etc is been robbed away and one feels that nothing is meaningful. This is vairagya. It happens. It produces fear and insecurity. When one jumps into this insecurity, one realizes that one is most secured. Slowly the truth unfold, and one gets clarity. After passing through this phase one becomes steady i.e. mind becomes steady.
2 ) Neti-Neti (Negation, Self Enquiry)
This is actual advaita, as taught in Yog-Vasista and Sri Ramana Maharshi. There is no OM. One thinks of Brahman constantly. e.g. is Nirvana Shataka by Adi Sankaracharya. Some 10 verses on Brahma Bhavana are given in Vivek Chudamani. In this type of sadhana, one enters into samadhi knowingly whereas in case of OM, one enters achanaka . But the result is same.
The 32 Bramha Vidyas also teach techniques to realize Brahman and can be referred here 2.
There are also few Gita's like Astavarka Gita, Ribhu Gita, and Shiva Gita that teach methods to realize Brahman/Self.
You are neither earth, water, fire, air or even ether. For liberation know yourself as consisting of consciousness, the witness of these five. If only you will remain resting in consciousness, seeing yourself as distinct from the body, then even now you will become happy, peaceful and free from bonds.(Astavarka Gita 3-4)
Meditate on yourself as motionless awareness, free from any dualism, giving up the mistaken idea that you are just a derivative consciousness; anything external or internal is false.. You have long been trapped in the snare of identification with the body. Sever it with the knife of knowledge that "I am awareness", and be happy, my dearest.(Astavarka Gita 13-14)
Listen to the methods of realizing such Atman. By doing Sagunopasana (worship of God in form), one should learn concentration. Then as like as Sthoolarundhati viewing rule, one should establish himself in the inward concentration and realize the Nirguna Brahman who is Myself As Atman in all.(Shiva Gita 14.6)
Atma bodha, Sri Shankara says
विविक्तदेश आसीनो विरागो विजितेन्द्रियः । भावयेदेकमात्मानं तमनन्तमनन्यधीः ॥ ३८॥
One should sit in a solitary place with a mind free from desires and controlling the sense organs, meditate with unswerving attention on the formless Atma which is infinite and one without a second.
In the book, self-enquiry, Bhagavan Ramana Maharshi
D: How is one to think of the Self?
M: The Self is self-luminous without darkness and light, and is the reality which is self-manifest. Therefore, one should not think of it as this or as that. The very thought of thinking will end in bondage. The purport of meditation on the Self is to make the mind take the form of the Self.
Self-enquiry is not the mind’s inspection of its own contents; it is tracing the mind’s first mode, the ‘I-thought’ to its source which is the Self. When there is proper and persistent enquiry, the ‘I thought’ also ceases and there is the wordless illumination of the form ‘I-I’ which is the pure consciousness. This is release, freedom from bondage