Lol the formula is super simple. It's really a case of picking a state and a creature. If your serious about writing Supernatural, I suggest you buy John Winchesters journal and their official monster guide for the show. Or just look up legendary folklore or your own knowledge of them. There's many well known ones, demons, ghosts, werewolves, vampires, zombies, wendigo's, bloody mary, angels, hell even make up your own! Go on some websites and look up lists for Supernatural creatures. Find out what they're known for and have a mysterious death at the beggining which no one can explain, your character becomes aware of it and through out the story tries to figure out what did it as more deaths happen whilst they do so. Maybe meeting Cannon characters on the way, then eventually having a big old show down and saving the day! Thats mainly in all SPN episodes, its just what you do in between~!
Go on Google images, find a map of US, pick a state, then pick a Supernatural Entity, Get info on it, create a death and have your character get involved. Simple as
Hope that helps!
Also, coming up with the name of the Fiction I either go with what the story is about, or use titles from Classic Rock songs like the show does~!