Self-inquiry is simple. It does not require you to do anything, change anything, think anything, or understand anything. It only asks you to pay careful attention to what is true and real. I have two sons. When they were about four, they both went through a phase of having nightmares. I would go into the room and switch on the light. Two small eyes blinked at me from the corner.
"What's the problem?" I'd ask.
"Daddy, there's a monster in the room," a timid voice would reply. Now, I had more than one choice of how to respond. I could tell my frightened boy that it was not true, there was no monster, go back to sleep. That response is equivalent of reading a book that says, "We're all one, there is no problem, just be with what is." Fine ideas, but they don't help much when your feeling scared. I could also have offered to free the monster cookies, talk with the monster, negotiate. That approach is like some kinds of psychotherapy. Treat the problem as real, then fix it on its own terms. But the only real solution I ever found with my munchkins was to have a good look. Under the bed, in the closet, behind the curtains, we undertook and exhaustive search.
Eventually my son would let out deep sigh, smile at me, and fall back to sleep. The problem was not solved but dissolved. It was never real in the first place, but it took investigation to make that a reality.