Think of something you really do fear, something that squicks you every time. Create a concept for a horror movie around it. Have a villain that would scare people when they see it in a movie poster for your movie. Give your movie idea a beginning, middle and then decide on what kind of ending it will have. Will it trail off and leave a question of more to follow? Will it end well, a happily ever after or does it end badly, leaving no hope?
Knives, sharp things and cutting of skin squick me. I don’t know why, but I feel a shudder inside when I see people playing with sharp things or a TV show where surgery is demonstrated. I’d make my horror about a knife throwing act in a circus, or a couple who perform as magicians for parties. Something happens and they realize they can’t trust each other. Or, the hand of the knife thrower isn’t as steady as it once was when they were younger. Something that changes the original situation and makes every throw of the knife a risk. My ending would be something blunt and final. The movie poster would show the knife thrower as the villian, his eyes focused ahead of him and his hand ready to throw a knife but with sweat on his brow, making him seem determined and not very caring about making a mis-throw.