The instructions that Sri Ramana Manharshi gave for practicing Self-enquiry were based on his own experience. When he advised other people to perform enquiry, he was telling them to do what he did when he realized the Self.
Sri Ramana practiced enquiry only once. It happened spontaneously when he was 16 and lasted at most a few minutes. Some authors refer to this episode as the "death experience" because Sri Ramana thought he was dying, but we call it the moment of realization because the most significant thing about the event is the fact that he became enlightened.
We use the word "moment" because Sri Ramana once remarked:
They say I gained realisation in twenty-eight minutes,
or half an hour. How can they say that? It took just a moment.
But why even a moment? Where is the question of time at
1. Reddy, N. Balarama. My Reminiscences. Sri Ramanashramam, 2012. 75.
Since Sri Ramana's use of Self-enquiry was spectacularly successful, it's natural to wonder exactly what went through his mind during those few minutes. If we knew the details, maybe that would help us know how to practice.
There are four main sources of information about the episode:
A short section in Self-Realization, B. V. Narasimha Swami's biography of Sri Ramana.
The notes that Narasimha Swami made of interviews with Sri Ramana for the biography. Some of these notes survive, and they contain details that he omitted from the book.
A short section in Sri Ramana Leela, Krishna Bhikshu's biography of Sri Ramana.
A teaching statement from one of Sri Ramana's writings.
These items are reprinted on the next four pages of this article.
David Godman has published an extremely detailed analysis of this literature on his blog with numerous citations of related passages in other texts. Both his article and his readers' comments are worth reading. For links, see below.