Create your own world for the story. Even if it's in a historical setting, a fantasy book can have elements from nearly any mythology and any kind of technology. The only rule it has to be consistent. For example, if some characters fire laser guns, you will need a good explanation for why another character is using a Colt 45.
Write a detailed sketch of the setting. Think about the technology, supernatural forces, culture, political structure, and appearance of your new world. Explain the "rules" of this world. If it has magic, does the average man on the street know about it, and if not, why not?
Create characters and come up with back stories for each of them. Describe the appearance, goals, flaws and goals of each character. This process may help you determine which one should be the main protagonist, and how these characters interact with one another.
Write out a plot outline. The outline can include actual dialogue, but it should just describe what should happen in each chapter, or stage of the story. Think about the role each scene plays in the overall story. Remove anything that does not "move the plot along" before you begin the writing process.
Plan to write at least twice a week for a few hours at regular times. A set schedule will help you focus on the book.
Write a first draft without regard to fine edits. While the first draft should be as clean as possible, the important thing is to see how the story flows and if all of the characters are properly developed. Read it with a critical eye, and make revisions as necessary for the second draft.
Restructure the novel as necessary. Set a new schedule and take the time to rewrite anything that doesn't feel right.
Do a third draft to correct any last-minute mistakes. This version should have correct grammar and punctuation.