Determine what type of horror screenplay you want to write. A horror screenplay can cover ground from a traditional ghost story to a slasher film. It can be gory or it can be a subtle psychological thriller. Determine what the story will be about and who will be in the story.
Choose the main character in the story. Determine what she will want in the story. For instance she will want to protect herself and her family from a serial killer or some other force of evil. Base the plot around the character's wants and needs and the obstacles she will face in pursuing them.
Create a hook that will introduce a fright factor. The hook sets up the film and raises the stakes. For instance, the hook at the beginning of Jaws includes the shark's first attack.
Build the story's tension after the hook. This can involve a mystery of some kind that will force your main character to try and solve it. For instance, if the story takes place in a haunted house, revolve the mystery around what is happening in the house and why. Pace the mystery so that each plot point ramps up the tension toward the midway plot point.
Determine what the midway plot point will be. The midway plot point occurs when the main character will have to make a decision. For instance, the hero realizes the strange occurrences are caused by ghosts and realizes he has to rid his home of the ghosts. In another example, the hero might discover that the serial killer is his best friend and now must confront his friend. The midway plot point will lead toward the climax, when the confrontation occurs.
Determine how the film will climax. Decide how the film's hero will confront evil. Set up the story so that the climax is inevitable but will create a showdown that is suspenseful.