First...it's a myth...one of those self improvement/self help myths based off a flawed scientific study parroted by the self improvement industry.
The human brain does not work by practicing or studying as most people currently do.
The human brain has two functions, a logical, analytical function and a sporadic, creative function....which doesn't work at the same time. The logic function works on intention, the creative function is subconscious and can be sparked by suggestion.
Great artists and scientists had work flows that switched between both functions.
They will work on something for 30 mins...go for walk or take nap and come back to it later...like days later....which is how the brain works best. By that time, your brain has worked on the problem.
Working on anything over an hour is pointless.
They didn't sit and "practice".
Becoming a "master" in a craft has more to do with talent, coworkers/contributors and work flow than doing something for an arbitrary number of hours.
It's a very American way of thinking...the whole "practice" and "working hard"...."practice makes perfect"
No it doesn't. That's not how any great artist or scientist actually worked.
If you wanna learn how to learn, Google Learning How to Learn.