Horror stories represent a collision between what we know for a fact is possible (death, plague, dismemberment, kidnapping) and what we believe is impossible (resurrected corpses, mutations, necromancy, ancient curses). What drives the narrative of these stories is as much a pulse-pounding urgency to protect (a family or community, for instance) as an overpowering sense of helplessness and confusion that the threatening forces might become too strong to conquer. The horror genre encompasses a slew of subgenres: ghost stories; witchcraft and sorcery; historical, regional or ethnic folklore; science gone bad; and psychological terror.
When planning your own horror story, first identify the elements that terrify you the most. These may be things such as drowning, burning, falling, suffocating, being devoured by wild animals or stung to death by bees, awakening in the middle of surgery or being buried alive. Second, imagine watching such horrible things happening to your child, spouse or best friend and being unable to do anything about it.
We often think of horror stories unfolding in places where people ought not have trespassed in the first place---a creepy mausoleum, an abandoned mine shaft, a haunted mansion, a cave, a fog-shrouded moor, a mortuary. While perfectly serviceable backdrops for unleashing your tale of terror, these might cause a disconnect with readers who could never imagine having put themselves in such places. To counteract this, experiment with more familiar and accessible locations.
Study horror masters such as Stephen King, who can turn an innocent hometown suburb into a devil's den. Think about the places that you frequent---coffee shops, movie theaters, gas stations, your child's school. Imagine what types of monsters, human or otherwise, might find these particular venues attractive places for seeking victims. Picture such seemingly innocuous settings on a dark and stormy night. Imagine yourself trapped there. Consider how the physical location or the materials used to construct it could trigger an encounter with the supernatural. Imagine the setting as the last place standing after a nuclear holocaust.
When you're at the top of the food chain, you don't ever think anything on the lower rungs might be planning an attack on you. In horror stories, the characters often find themselves lulled into a false sense of security. They believe they're in charge. But that's when things get ugly.
Take a moment to look around you. Identify six random objects and consider what they could do to harm you if they suddenly came to life. If you're in the kitchen, you're probably in the most peril with all those sharp, pointy, heavy and electric objects. If you're in a home office, you're not only at the mercy of technology running amok but also of scissors, letter openers, pencils and books. Rushing outdoors could get worse, especially with every plant, rock, bug, bird and squirrel.
The two primary objectives of protagonists in a horror story are (1) to get away and/or (2) to destroy the enemy. Unfortunately, aspiring horror writers often fail to ascribe a comparable level of motivation to the bugs, beasties, zombies or whatevers on the attack. While a horror plot does not need to strive to make its villains sympathetic, it should still treat them the same way it would human bad guys, giving such entities plausible reasons for their actions. Some of these reasons might include revenge for past transgressions against them, anger that their eternal sleep has been disrupted, chemical imbalances, the depletion of food sources or the quest for companionship. As for those No. 2 pencils that suddenly come to life and start drilling you with lead, maybe it's just because of built-up resentment that you've been spending too much time at the keyboard.