Von Neumann did it by watching loads of Khan Academy videos on YouTube, taking regular Vitamin D supplements, using Anki cards and practicing mindfulness meditation.
Have a lot of money, be privileged to work with lots of very smart people, and spend a lot of time learning from the sound of this article.
I'm always super-suspicious when I hear someone's brilliance described in anecdotes. Some people are better at promoting themselves than others. So perhaps judge him only by his published work and not his reputation.
By the age of six Von Neumann could divide eight digit numbers mentally and speak in Ancient Greek. If it is nurture, it is something accidental which happens in early childhood, not something you work on and learn about later in life.