Nirvikalpa means being without any changes. When one achieves this Samadhi, he literally becomes God. He becomes one with ultimate. It means your whole being, your whole identity is dissolving into Cosmic existence. The individual inner space has no more disturbances, differences. Best direct experience of Nirvikalpa Samadhi can be achieved by unclutching. Please understand, unclutching is not a path leading you to Nirvikalpa Samadhi. It is a process directly leading you to Nirvikalpa Samadhi. The moment you start unclutching you are falling into Nirvikalpa samadhi.
It is not a peak to attain, it is a valley to be fallen into.
What steps, what tapas, what kriya need to be performed to attain it ?