Based on the preface, what is the fundamental principle for attaining salvation, and how should one approach a Guru to receive their grace?

The preface to the Shri Sai Satcharita establishes that the fundamental principle for salvation is securing the grace of a Sadguru, a belief that has existed since Vedic times. To receive this grace, one must serve the Guru with their entire being. Citing Dnyaneshwar Maharaj, Chapter 0 advises surrendering at the Guru's feet with body, mind, and soul, and performing all service without ego. This selfless service is presented as the doorway to attaining the treasure of knowledge that dispels all doubts and enlightens the heart.