Create characters your readers can identify with first. In most cases, put ordinary people in extraordinary circumstances. Aim to create characters with faults. Perfect characters rarely make interesting characters. Give your characters a life beyond the life in the story. Give them careers (preferably you can tie in with how they survive the terrors you place before them), friends or enemies, and a personality that readers will love or hate. Characters in a gray area are rarely successful.
Write a storyline that presents a challenge to your characters. Regardless of the horror in your story, the story and the actions of the characters must be realistic within the context of that story. This includes how your protagonist (main character) overcomes the terror.
Place gross-out scenes in your story strategically. A gory story will introduce bloody, steaming entrails being drawn from a corpse. A gory story will detail a knife slicing into flesh. There may be a scene in which a creature punches its fist through someone's chest and removes his heart. Place these descriptions in scenes where they belong and detail within the context of the scene. Throwing a gory scene in out of context isn't the key to a successful story.
Pace your gory scenes and make them progressively more twisted. If you write one gory scene after another, pretty soon your readers will become immune to them. If you give the first gory scene in your story all there is to give, then creating a scene to top the previous scene will be difficult. You run the risk of forgetting the story and writing to outdo your last gory scene. Balance subtle frights with gory scenes to help keep your next gory scene impressive and stomach churning.