Think about the places, things and people in your story in an obtuse manner. What lurks in the abandoned school? Could that lawnmower start by itself? What is the old man next door growing in his garden? Focus on that one key element that you can use to turn on its head. Once you have this element, it becomes the spine of your plot and the rest of the story falls into place.
Create a believable villain (antagonist) by giving him a reason for creating mayhem and murder. Most antagonists in horror stories have deep psychological problems. This usually fuels the engine of the plot. Spend time researching and developing your antagonist's torturous past. It anchors your plot.
Develop your hero (protagonist) and a cast of supporting characters by placing them into the situation that you created and letting them react to it and one another. The protagonist usually has an entourage of friends who can be used not only as potential corpses but as plot devices to establish an emotional bond between the protagonist and the audience.
Write a logical conclusion that provides the climax of your plot in a believable showdown between your story's antagonist and protagonist. Satisfy your audience with a conclusion that fits the characters. It's all right to have an open-ended conclusion, especially if you plan on creating a sequel.