Requirements of Flow
1. Your activity has clear goals and provides clear feedback.
2. You believe that your skills are a good match to the challenges of the activity, giving you a sense of control.
3. You are intensely focused on what you are doing.
4. You lose awareness of yourself and the passing of time.
5. The experience becomes self-rewarding. 
And regardless of the culture, regardless of education or whatever, there are these seven conditions that seem to be there when a person is in flow.
There’s this focus (1) that, once it becomes intense,
leads to a sense of ecstasy (2),
a sense of clarity (3): you know exactly what you want to do from one moment to the other;
you get immediate feedback (4).
You know that what you need to do (5); it’s possible to do, even though difficult,
and sense of time disappears (6), you forget yourself, you feel part of something larger.
And once the conditions are present, what you are doing becomes worth doing for its own sake. (Intrinsic motivation, 7) 
 Edited from Susan K Perry, Writing In Flow: Keys to Enhanced Creativity, 9.
 Mihaly Csikszentmihalyi: Flow, the secret to happiness, TED Talk https://www.youtube.com/watch?v=fXIeFJCqsPs