For Shohei Ohtani, his father was the first person who taught him baseball, and during his youth, also served as his manager and coach.

During his Little League years, Ohtani and his father kept what was essentially a shared “baseball notebook.”
His father, Toru, would write evaluations and advice from each day, while Ohtani would write reflections on his performances and identify areas to improve.

What mattered most was not simply pointing out mistakes, but thinking about what could be done next to overcome them — and then actually taking action.
As is often said in the business world, the key after failure is not assigning blame, but identifying the cause, finding ways to prevent the same mistake from happening again, and putting those solutions into practice.

Another important habit was recording failures.
Mistakes are often met with scolding and then forgotten, but that alone leads nowhere.
By writing things down, Ohtani and his father made sure that what needed to be done stayed clearly in mind.

On most pages of the notebook, three basic principles were written again and again:
to play energetically with a loud voice,
to practice catch with full effort,
and to run hard.

“These three teachings were very basic,” Ohtani says, “but I still remember them. I think they are things you’re told no matter what stage you reach.”

They may not be flashy, but they never change.
Those fundamentals remain at the core of who Ohtani is, even today.

Source

This quote comes from a Japanese book published in Japan and is not currently available in English.

Opening a Path, Crossing the Ocean: The True Story of Shohei Ohtani, p.93