Consider what kind of story you wish to write, how you wish to write it, from whose perspective or something more omniscient, and what you want to accomplish throughout the book. Also, think about how you enjoy writing. Are you more of an architect? Do you enjoy engineering each scene and story progression before beginning? Or are you a gardener with a more general idea of how the story will grow as you write it? Think about the significance of your story and what meaning you want to impart through the plot. Will this be a story of good verses evil or something more complex? How will you begin the novel, with background or jumping right into it? Who will be the important characters and how well can you convey their relationships to others within the story? How descriptive will you be? Will you entice the senses or will you leave more to the imagination?
In essence, I would begin with story-boarding, personally. Or perhaps a writing exercise based on getting out the elements you want to incorporate into the novel and seeing how they look on paper. Get a good idea of how the novel will progress as you write it, connect ideal scenes you want to write, and then sit down to have at it. It doesn't have to be perfect and will likely undergo numerous revisions, but just the act of writing will do you good for a bit of practice regardless.
Best of luck on your quest and feel free to update here as you need.
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