I literally just go for it.
Usually I'll have an idea cooking up for a few days, and I'll begin putting it into small notes, for example, "MC is an Astronaut and due to malfunction finds herself lost on a strange new planet". I'll keep building it, then I'll go into more detail on the main character. And really... it just keeps going, I'll pick a time and I'll just start writing..
If I'm having trouble setting up the book I'll just look at actual examples of stories. Whether it's a fanfiction or not.