Research the literature. Get an idea of what has already been done by different writers. Let yourself be inspired by the work of others. Notice what devices of the genre are overdone, and think about how to either avoid them or twist them to create a fresh approach. Read up on not only modern vampire stories, but older sources as well such as Bram Stoker's Dracula and J.S. LeFanu's Carmilla. Investigate vampire myths from different cultures.
Identify what intrigues you about the vampire myth. Think about your own personal reactions and interpretations of vampire stories. Which stories do you find most fascinating and why? What questions do you have about vampires that remain unanswered by the books you have read? Think about genre and tone--does your personal vampire universe emphasize romance, humor, sex or mystery? Consider what characters, settings and plot events would help illustrate the aspects of vampirism you find most compelling. Follow the advice of Nobel Laureate in Literature Toni Morrison, "If there's a book you really want to read, but it hasn't been written yet, then you must write it."
Let your imagination flow. Do not censor yourself. After letting images of fangs ferment in your head for a while, follow the thick red vein of creativity. Give birth to your living and undead characters on paper or screen. Nurture them with a rich sense of time and place; challenge them with plot twists and turns. Some writers start with an outline and others let the characters determine how the story evolves. Discover which writing style is truer to your creative impulse. Remember what Sylvia Plath said "the enemy to creativity is self-doubt."
Stay committed to your story. Avoid giving up on the days the words do not flow easily. As Nathaniel Hawthorne wrote, "Easy reading is damn hard writing." Even if it seems imperfect, write all the way through to some sort of ending or resolution. You can always revise and polish a finished product but a story you never complete remains nothing more than a dangling thread.
Refine your story. Through your own rereading, and through the comments of trusted first readers, look at the story's strengths and weaknesses. Do your characters seem vivid or like stereotypes? Does the plot carry readers along, enticing them to read the entire story? As Elmore Leonard said, "I try to leave out the parts that people skip."