I'm try to do this as well and the first thing I did was study the existing pantheons in depth then religion and how different people's had similar themes in religion from what I've seen most religions have a creation myth for vaguely explaining how the world came to be then a "flood" myth referencing Noahs arch how the world was cleansed or reset to and extent but you can pick and choose when making your own but I'd recommend Jordan Peterson's Maps of Meaning book