It is a truly exciting feeling. Some might even say, erotic.
It all starts with routine. I know I say that a ton. If you use smells, like a certain perfume, or candle when you start writing then you mind will know it is time to go to flow. However, you can use routine too. Everyday at a certain time you get up you write. You have lunch you write. Whatever routine you chose stick to it. That will strengthen your connection to the almighty flow. It is like any muscle the more it is used the more readily you can get into it.
What process do you use to get into flow?