You don’t train to become a mentor before it happens, it just happens.
All the mentors I’ve had, they never took any courses to become a mentor, it just happened.
People saw them doing their work and thought “I could learn a lot from them” and then they want to learning from them.
Mentors aren’t born in the proverbial sense. The first time someone asks them for advice, guidance, and suggestions – they are what happens next is showing up each day thereafter and hoping their guidance gets passed on to the Mentee.