In a word? Starting.
Starting is the absolute hardest part about writing a book. Everything after that is easy, even if parts can be frustrating or annoying.
So how do you start a book?
Give yourself permission. You don’t need permission from anyone else. Just yourself. When you’ve done that, sit down and write… and write… and write. Even if you trash it later and start over, you have to write. Without the words there is no story to share.
For me, I wanted to write Zaria’s story for a while. I knew Zaria Fierce as a character, but I didn’t know her story. I did a lot of world building for a school of misfit fairy tale children and archived it all, including the prologue and first chapter. Then I wrote her in a Beauty and Beast fairy tale setting where her kingdom was under attack. Hm… not that plot either. So I put those chapters aside.
What I did next was write about Zaria. I wrote her description, her character, and her background. She became a younger character at age 12 instead of age 16. It was amazing how quick it poured out of me. But I hesitated to keep going… I had two previous false starts. All I had now was Zaria. Who was the villain? What was the plot?
I set the story aside for a few months and returned to it when I found the villain. I ruminated about him for a long time. What was his name? What was his goal? Would he have powers and what would they be? When the villain came to me, he arrived fully form as a particularly nasty river-troll with an agenda. When I had Olaf the world built itself, but I would never have found the villain without having Zaria fleshed out.
It doesn’t matter where you start or how you arrive at your story. What matters is that you tell it.