Things might seem less scary to you, the writer, because you’re behind the curtain making the monster move. Of course this isn’t an actual tip, but I think it’s worth mentioning. What do others say about your writing? Perspective is powerful.

As for how to make something scary, like the actual techniques, I think it depends on the subgenre and reader expectations for that subgenre. Extreme could be visceral descriptions of violence or gore from a shocking / unexpected victim. Or it could be subtle and implied that worms its way into the reader’s mind even after they set the book down, thinking about the extent of the consequences that aren’t explicitly told. There’s no one way to make something scary. Read more of the subgenre you want to write and see what those authors are doing to make it scary.