So I am working on writing a campaign set in the world of Stephen King's stories. I'm trying to figure out how to homebrew his monsters and other villains but I'm having a tough time.

I'm trying to make The Crimson King, Randall Flagg, Pennywise, Cujo, Annie Wilkes, Gage Creed, Christine, Mr. Gray, the Langoliers, and the creatures from The Mist. Only thing is I don't know what to do with it. Could anyone give me some help?

What are you struggling with? There are so many parts of monster design that it's going to be impossible to help you without more info.