Read great fantasy books. You must know the genre in order to be a master of it. "The Lion, the Witch and The Wardrobe" and all the books in the "Harry Potter" series are recommended. Some other great fantasy books include "The Wonderful Wizard of Oz" and the "Twilight" sagas. Fairy tales like "Cinderella" also fall into the fantasy category.
Write an outline for your book. Because of the nature of the genre you are writing within, these outlines can be creative as well as factual. Include a clearly stated beginning, middle and end. State how many planned chapters there are for the story and describe each chapter.
Write a 2- to 5-page description of the fantasy world in which your story takes place. Since it is a fantasy, it's a great idea to do a separate outline for the fantasy world. Write the physics, facts and history of the fantasy world. Include heroes, triumphs and failures within that world. You should feel as though you know that world fully before writing about it.
Make a character list. These names should be filled with magic and meaning. If you choose ordinary names, there should be a reason that is stated in the book. People expect something different from the fantasy world. Perhaps the name Angela is used for a character who can communicate with angels, or an average name could be used on someone who goes from his normal life into a fantasy one. This will be added and subtracted from as you go. It's important when writing to keep track of this. It will keep you focused on the fictitious people who are now filling your written world. A short description of each is recommended if you have a co-writer, but you can keep descriptions in your head in most case scenarios.
Write the first few chapters. Compare what you've actually written to your outline. You might want to modify either the text of the book or the outline itself to accommodate differences between the two. You will get a better idea of whether you can write on the planned track.
Continue to compose a few chapters at a time. Compare and contrast to the outline. Keep track of your character list, making changes as you go.
Read and rewrite briefly once you have written the entire book.
Copyright the book after your first version. Yes, you will make drastic changes, but your original copyright will still be enforceable. You should copyright the final draft and the first version. This can seem silly, but it's important. Although you probably trust your own judgment, all humans are imperfect. It's worth it to take this extra step. See the Resources section for the national copyright website. You should also consider registering it with the Writer's Guild of America, especially if you are writing a sister script. That link is also located in Resources.
It's important to get a second, third and fourth opinion. It's equally important to choose only readers whom you know very well and trust implicitly.
Rewrite your fantasy novel. A great chunk of writing is actually rewriting. Enjoy this part of the process. Unless you are planning sequels, this is your last chance to spend writing time with these characters you worked so hard to create.