Know Your Readers
The first thing you should know about writing for any genre, including horror, are the readers. Understanding why the readers read that genre is the key to writing good fiction for that genre. Horror fans are thrill seekers who love to be frightened and deeply disturbed by what they read.
Know the Genre
The discovery of severed hands and other body parts in an old house is not fear, that’s just plain gross. The fear is the question of who hacked these parts off and the tragedy is the character going up stairs to investigate a noise from there after finding these body parts instead of running away from this house. The two ingredients in horror is fear and tragedy.
Read books written by writers of this genre: such as Stephen King , Dean Koontz, H.P. Lovecraft. Horror is about fear and tragedy and rather one is capable of overcoming that. It’s not all about vampires and so on. Just because a supernatural creature is in a story doesn’t make it a horror story. Although I will admit there is horror in J.K. Rowling’s Harry Potter series because of her scary death eaters (I wouldn’t want to run into one of them in a dark alley) but her series is fantasy not horror.
Build the Plot
First think of something that would scare you and then build from that. Keep writing. Remember a writer writes always.
Create the Characters
Create realistic characters that you and your readers can relate to. Give your characters a past, a present and a future . Give them both negative and positive traits because no one is perfect and your characters shouldn’t be either in order to relate.