I once heard the difference between these three facets of the horror genre described thusly: “Fear is knowing that a werewolf is hunting you. Terror is when you see it and it charges at you. Horror is realizing that your feet are stuck to the floor.”
Obviously, good horror manages to spread a mixture of all three of these responses throughout the story line. But with the right amount of tweaking, added details and imagery, this brief description is also the perfect example of one singular, escalating event.
Put a character in a situation (not the werewolf one) which gives them fear, and force them to linger in it for a bit. Make sure to play with pacing and subtle details that will add suspense. Then give your character a reason to feel terror: raise the stakes of the situation and send them into a panic; have them react to the terror. Finally, make the situation go from terrifying to horrific. What is the worst that could go wrong? Make it go wrong. Give your character a reason to believe they’re not getting out of this situation.
Now, how will you end it?
(Bonus Challenge: make the situation realistic instead of supernatural. Extra Bonus Challenge: make it as close to ‘normal’ as possible. What in our daily lives could possibly escalate to horror?)