Work out some basic ideas regarding your fantasy story. Good fantasy tells a tale of heroism, science, history or myth, but with an element of magic or mystery that will engage the reader and take them to an unfamiliar world.
Think back to tales from your childhood that you never quite believed, but wished you could. Now think of the human elements of that story that you connected with, despite the unbelievable aspects. Human emotions like these should be the basis for your tale, even if humans do not play a part in it.
Think of characters, motivations and reasoning to support your story. While fantasy is a wide-open genre, all stories must connect with their audience, and good characters will help you do this.
Expose yourself to classic as well as contemporary fantasy. This will help you understand other writers' successful approaches to the genre. Be careful not to repeat themes or storylines from other books when you move on to the writing process.
Write absolutely everything down. Character sketches, story elements, rising and falling action, and character motivations will come to you away from your computer as well as in front of it. The best ideas can be those that you lost, so record everything, even material you don't think you'll use. If a character or detail won't fit into your current story, they may have a place in your next one.
Purchase a note pad or a voice recorder to capture good ideas before you forget them. Once you record them, type them all out later so you can cut and paste them into a discernible outline.
Make a habit of recording or writing down names or titles that inspire you. The names of characters and places in fantasy writing range from brilliant to laughable, so make sure you're not coming up with them on a whim.
Develop your characters as if you knew them personally. Write out detailed character sketches, including age, appearance, demeanor, an elaborate backstory and their feelings toward other people and places.
Create rules for your world. If you plan on including magic, it must have its limitations. If a stroke of a wand could solve your entire plot, it can make your story look silly and pointless. If you plan to write outside of the known natural world, you must bring a natural order and internal logic to what you create.
Consider making a map of your world. While this seems a bit cliched, making a map of your story's universe will make it seem real to both you and the reader. While most readers will be able to visualize the miles between Mexico and Ireland, few will be able to discern the distance between the dreaded Mountains of Sorrow and the enchanted Lake of Shimmering Sleep without a reference.
Create your characters to be interesting in their motivations and to interact with each other in dramatic ways. A good environment is nothing without characters to fill it.
Motivate your characters with human emotions. Whether you're using human beings at all in your story, it's more than likely that your readers will be human, and you'll need to use human motivations to explain why your characters do what they do.
Try to keep your characters free of archetypes and stereotypes. Having women evolve from maids into fighters is a cliche of fantasies past. Your chances of creating something original rest on your ability to make your characters unique.