Did Baba's Guru teach him through mantras and rituals? What was his method of instruction?

No, Sai Baba explicitly stated that his Guru did not rely on traditional mantras. He explained in Chapter 19, "my Guru did not blow any mantra into my ears, so how can I blow one into yours?" The Guru's method was far more subtle and profound. Baba compared it to a mother tortoise who, from one riverbank, nurtures her young on the other sandbank with her gaze alone. This loving, merciful glance was the source of all protection and spiritual nourishment, rendering the 'effort of a mantra' useless in comparison. The core of the teaching was this silent, loving attention.