Create a cast of characters. Your protagonist might be a vampire, or she might suspect someone in her family or school is a vampire. If she's a vampire, the antagonist might be trying to expose or even destroy her. Tween novels typically have a teenage protagonist, someone slightly older than the teen, but not too old to be relatable. The protagonist should have a clear, primal goal, which will keep her engaged with her journey.
Develop a story. In Richelle Mead's "Vampire Academy," two girls, one half-human/half-vampire, the other a mortal vampire princess, must survive the peer pressure and other difficulties that come with school. Other books, like Stephenie Meyer's "Twilight" series, center around a romance. Bella's forbidden romance with the vampire, Edward, creates tension and suspense, but stays chaste in its details for the tween audience.
Outline the novel, creating at least three acts. The first act sets up the story, introduces most of the main characters, and gives an inciting incident, which sends the story in motion. For example, your main character might discover he comes from a long line of vampire hunters, and he must choose whether to take up the cause. The second act delivers the meat of the story, where your protagonist must overcome obstacles on his journey. The third act contains the climax and resolution to the tale, forcing the protagonist to face off against the antagonist. Details of the outline will most likely change during the process of writing, but the spine of the story should hold up. The structure needs to be solid for the novel to succeed.
Create a writing schedule and stick to it. Stephen King suggests writing 1,000 words a day, which would mean an 80,000 word book would take 80 days of writing. Breaking the book into smaller goals helps keep you on track and takes away the oppressing magnitude of writing an entire novel.
Take at least two weeks off before rewriting in order to gain some perspective on the work. Check out other tween vampire novels to see places where your novel might be too similar. Rewrite those sections completely or simply add a unique twist. In addition to punctuation and general grammar fixes, pay attention to your characters. Are their motivations clear? Are they believable? Characters need to feel fresh and fully alive. In the vast library of vampire novels, certain rules have been established, like a long life and a thirst for blood, but you need to find special ways to deliver these rules in order to separate your book.