Devise a world in which to set your fantasy novel. Typical stories in the genre take their cue from J.R.R. Tolkien and/or European fairy tales. Their worlds feature kingdoms of peasant villages, haunted forests filled with terrible creatures, wizards practicing eldritch magic and heroes tasked with fulfilling mighty deeds. To make your world stand out, think of something that makes it unique. Don't just ape "The Lord of the Rings"; find some new way to expound upon the clichés. Perhaps the fairy-tale kingdom exists with Wild West-era technology, or features creatures that no one has ever seen before. Elves and orcs are a dime a dozen in fantasy novels; to write a great one, come up with something new.
Work out a basic storyline for your novel; it need not be anything too detailed at this point, just a basic notion of the plot and a sense of where you want it to go. Fantasy novels typically embody a variant of the Hero's Journey, in which an untested youth embarks upon a perilous quest to destroy the forces of evil. Along the way, he finds companions, gains new powers and shapes the destiny of his world. As with Step 1, the basics have become extremely clichéd; to tell a great story you need to find some new variation. Michael Moorcock's "Elric" series is a great example: instead of an untested youth, his hero is a decadent king who loses everything he has and may embody the very evil most fantasy heroes seek to destroy.
Write up an outline of your novel, detailing the key characters, main events and overall structure of the story. Use this as a guide while writing the meat of it. You can depart from the outline midway through if the story takes you in another direction, but an outline provides a basic sense of where you're going and prevents you from rambling. Fantasy novels often take an episodic approach to the plot, as the heroes encounter challenges and obstacles on the way to their final goal. Look for incidents with a sense of flair or imagination, instead of just "fighting monsters" or "finding a magic sword."
Write the novel out from beginning to end. Don't stop for revisions and don't worry if it's not perfect. Get the entire book down on the pages in some form. Once you do that, you can fine-tune it to your heart's delight. Watch the way you build the drama as you write: Fantasy novels adopt a classic structure of rising action leading to a world-shattering climax. Maintain that if you can. You also want to keep the characters in the forefront; don't let them get buried beneath sword fights and similar plot devices.
Let the draft of the novel sit for a few weeks once you've finished it. It helps you decompress and gain some objective clarity for the revision process. You may want to avoid reading other fantasy novels in the interim; they'll cloud your vision with those of other authors and perhaps induce you to make changes that will make your book too much like theirs.
Go through the novel at least twice and perform revisions until it meets with your satisfaction. Correct grammatical errors, polish the text and rework any sections that don't flow the way you'd like. If you know an editor and have a trusted friend or spouse, let them look through it as well. Ideally, they should be fantasy fans or know the genre; it will help them better understand the work and make helpful comments.
Print your complete novel out and contact a publisher about printing it.