I must confess that I am not very familiar with horror writing and I don’t read that type of book. However, it seems obvious that the goal is to shock and frighten the reader.
This will take much creativity on your part. It will also entail a plot line that is weaving and in the end, most unexpected. For this type of writing, you will have to spend more time on your plot than with most other types of writing. You will have to decide when the plot will take a different twist and what the outcome will be.
You will also have to develop your characters and in some cases, don’t tell all so to speak. Unlike other forms of writing, keep at least one or some of the characters under a veil of mystery as the story unfolds.
Basically, you want to take your readers on the wildest ride in the amusement park. You want them to feel the hairs stand up on the back of the neck.