I do not think it is that hard honestly.
You start with an idea, and build from there. "Bob loves Sally, but Sally is part zombie part werewolf and wants to eat him" Then you give some personality traits to them , toss them in a scene and let it fly.
The difference is in the telling and the details. Star Wars, and Lord of the Rings are the same story, but you would never consider them to be copies of each other.