Consider the thoughts your character would go through when something horrible goes down.
You want what happens to be terrifying, but it doesn’t necessarily have to be horrible. What I mean by that is you can have a scene that’s scary, but in that scene, no one dies or gets hurt.
Give your readers a break from horror every now and then. This should go for every genre, and a great way to do it is by making sub-plots.
Make it realistic. Even if it’s about the zombie apocalypse, or something that can’t actually happen. The supernatural has it’s own set of rules, and you need to ensure that the story isn’t so far-fetched that it makes no sense.
Don’t add anything for shock value. Readers notice when you’re trying to be edgy.
Give your character something to lose. Gore and violence aren’t as terrifying to read as knowing your character can lose everything they love at any moment.
Get the reader invested in the character.
You want the reader to have questions, but don’t want them to be confused. So don’t reveal everything too soon, but don’t reveal too little. This will help add suspense
Learn how humans react in terrifying situations
Consider having your character think and act irrationally
And if you really wanna have fun, consider having your character slowly descend into madness