1. Write every day.
I would sum it up with another precept I wrote down when I was a teenager. It was from The Agony and the Ecstasy: “The most perfect guide is nature. Continue without fail to draw something every day.” Substitute the word “write” for the word “draw,” and that would be my advice. Just write. Don’t wait for the perfect moment: there’s usually no such thing.
2. Don’t think about writing a book.
Also, don’t think about writing a book. That can be a little daunting—even just the saying aloud: “I’m writing a book.” Instead, just think in terms of writing a page in a book. That’s all. A page or two every time you sit down to write. Before you know it, you’ve got yourself a chapter. Then a couple of chapters. And you’re on your way to writing a book.
Every author I know gives that first piece of advice, but I really found the second piece of advice extremely helpful.
Writing a book takes many long hours. It is a huge commitment. It takes a lot of excitement and motivation at the beginning of the project to push through the very difficult parts of the process.
So, just write. Write to the point that you can’t stop writing until the story is finished.