When you do it without thinking, effortless, and relaxed…
For instance, when I was skiing fast the most difficult piste. I was in the flow because every thought would cause me to fall…
But that is just crazy… We don’t need to make risky things to be in the flow…
When you can’t relax or meditate then at least you can learn to be in the flow … And that for so many things you can do, even if they are unattractive or boring… 
For instance:  
Assembly-line work All kind of sportsDancingCar or motorcycle drivingPainting even from wallsMaking music or listening to musicWritingSex
The catch is that you make the things in a specific rhythm. With a rhythm, it is so much easier to go in the flow because our mind is hooked with rhythm.
When the soldiers marched in tune with their drums, then nearly every soldier lost his fear and was in the flow…
You need to get that feeling how to be in the flow…
If you hear straight forward music like the March, Waltz, and then move in this rhythm your body… Or just march like a soldier or dance and maybe count the beat so your mind is occupied… 
Or I choose a mantra I repeat in 4-beat rhythm.I divide the mantra in syllables and even better with the sounds I drive the Kundalini upwards… 
For instance: “Huh Ohm Kali”
Huh (the ‘u’ like ‘oo’ in school)Ohm Ka (the ‘a’ like ‘u’ in Run)Li (the ‘i’ like ‘ee’ in deep)
And it works… try it out.
My Video: How to go in the flow? https://youtu.be/R73NobSoShYMy Audio: https://divinesuccess.net/wp-content/uploads/2021/Podcast1/How-to-go-in-the-flow.mp3