Sometimes you can’t. If the story is thin to begin with, then maybe it’s just meant to be a short story.

But most of the time it can become a full-length book by adding twists and turns and more characters to the plot.

Put obstacles in the way of the main character achieving what he/she sets out to achieve. Those obstacles will become the bulk of the story with the beginning of the story being the beginning of the book and the ending of the story being the end of the book.

It’s what comes in between that’s the difference between a short story and a book.