Start with the setting. Invent the world in which your character will live, fight and exist. Create a theme of your novel. For instance, a good vs. evil plot could work. Then include elements of magic and how the magic will be used throughout your best-selling novel. You can include romantic or suspense elements to your novel, but fantasy needs to be at the forefront.
Envision believable characters. For your best-selling fantasy novel you must have characters that actually have importance to your story. For instance, the main character could be a 21st-century person transported to a different world to fight against evil. Decide who possess the power and who is the enemy.
Create a guideline. The guideline is for you to know where elements like action and conflict will take place. This guideline is less for the reader and more to help your make your best-selling fantasy novel a reality. So use the details in your guideline to help you complete your story and ensure that it is consistent.