J. K. Rowling (born on this day in 1965) has sold a few books in her career. So it is somewhat refreshing to see her resisting the urge to lay down some must-follow rules for other writers to follow. In fact, in this piece from 2019, she points out that her breakthrough came largely from going in the other direction:

I found success by stumbling off alone in a direction most people thought was a dead end, breaking all the 1990s shibboleths about children’s books in the process. Male protagonists are unfashionable. Boarding schools are anathema. No kids book should be longer than 45,000 words…

Still, she presents a few common-sense rules of thumb to follow:

• Reading: “Reading is the best way of analysing what makes a good book. Notice what works and what doesn’t, what you enjoyed and why. At first you’ll probably imitate your favourite writers, but that’s a good way to learn. After a while, you’ll find your own distinctive voice.”
• Discipline: “Sometimes you have to write even when the muse isn’t cooperating.”
• Independence: “By this, I mean resisting the pressure to think you have to follow all the Top Ten Tips religiously, which these days take the form not just of online lists, but of entire books promising to tell you how to write a bestseller/what you MUST do to be published/how to make a million dollars from writing.”

That does not mean if you do everything she says, success will follow. Nothing is that simple. Remember what Dumbledore said:

We must all face the choice between what is right and what is easy.