There are no set rules—a writer defines their own. The easiest thing about world building is that there are no rules.
"There is nothing to writing. All you do is sit down at a typewriter and bleed," said Ernest Hemingway.
As a writer, you mustn't feel obligated to follow the direction of others.
Don't pay too much attention to what will sell. Write what's in your soul—throw your ideas, interests, and convictions onto the pages.
Believe in your creativity.
"Writing is magic, as much the water of life as any other creative art. The water is free, so drink," said Stephen King.
What matters in fiction writing the most is good writing that sharpens the suspense and delivers the goods to the reader. As writers, it behooves us to give the reader what they crave, when they want it, and if possible, introduce a twist that gives them more than they ever imagined.
"Description begins in the writer's imagination, but should finish in the reader's," said Stephen King.
Famous authors wrote what they wanted to write; creating worlds that no-one had hitherto written about. The hardest thing about world building, though is staying consistent in establishing verisimilitude, and convincing your readers that it's real.