Whenever I create horror (or really every game I make as I've discovered) I always start with a story. I always start with worldbuilding, lore, story, characters, and small specific details and moments. From that, I build the gameplay around it and try to make it engaging as possible and something I would want to play ( which is very important to me). After that, I start modeling everything and the game gets put together. I've found that this workflow works great for my games. Over the years (since elementary school) I've been writing stories and characters and I've been getting better, eventually reaching where I'm at now where I can write believable humans and believable worlds.