Think about what scares you. Any horror writer worth his salt writes about subject that he himself finds terrifying. If you don't feel the fear yourself, you'll have an extremely hard time conveying it to your readers. At the same time, make sure your fears are shared by at least a certain amount of society, so that they can share it more readily. (A fear of insects, for example, works much better than a fear of ice cream cones.)
Catalyze your fears into a concrete plot or scenario. What form do the fears take in a story? Are they a monster in the dark, a crazed killer or something inexplicable? How will they threaten or menace your protagonists? What steps (if any) can be taken to thwart them? When planning out your scenario, note how quickly or slowly it moves and how they might affect its readability. Also account for any unexpected twists you want to include, noting when they appear and how they alter the nature of the story.
Ground the fearful material into a plausible reality. Horror stories don't work unless the readers can identify with the situation. Make the setting a real place and provide characters who are as plausible and fleshed out as possible to grapple with it.
Establish the proper mood in your text as you write. This means establishing an appropriately fearful atmosphere, a tone of suspense or tension, an air of mystery that doesn't explain too much too soon, and a slow build-up of frightening material as you work towards your climax. Horror books generally need to move along briskly, so don't linger unduly on too many details if they feel like they're bogging down the pace.
Consider the use of violence carefully. Horror books are known for their descriptions of graphic horror and in the right amounts, a little blood and guts can be very effective. But in many cases, the violence simply covers up for a lack of ideas behind it. Use violence as a garnish in your horror book writing, not the main course.
Revise the book after you have written it to make it more polished. This includes a certain amount of copy editing, but also a critical eye on how your horror story flows. Look for places where the text has bogged down, eliminate extraneous or unnecessary passages, and make sure the internal logic of the story is consistent throughout. A good editor can help with these duties, but at some point, the writer will need to get in there herself.