How To Write A Horror Story???
I'm not sure that I'm the right person to answer the question "how to write a horror story", but I have a lot of people who visit on any forums looking for advice and guidance writing horror stories. So here is the suggestion I have for those who want to learn how to wite a horror story.
If you want to learn how to write a horror story, then you should read horror stories. And you should start with the classic, and analyze them to see why they work. Some classic horror stories to consider include:
"The Monkey's Paw" by W.W. Jacobs
"Dracula's Guest" by Bram Stoker
"The Masque of the Red Death" by Edgar Allen Poe
"The Picture in the House" by H.P. Lovecraft
That's obviously just a partial list, but the good news is that those are all public domain stories that are available to read online.