It is said that, during the fantasy book in the late eighties, publishers would maybe get a box containing two or three runic alphabets, four maps of the major areas covered by the sweep of the narrative, a pronunciation guide to the names of the main characters and, at the bottom of the box, the manuscript. Please… there is no need to go that far.
There is a term that readers have been known to apply to fantasy that is sometimes an unquestioning echo of better work gone before, with a static society, conveniently ugly ‘bad’ races, magic that works like electricity and horses that work like cars. It’s EFP, or Extruded Fantasy Product. It can be recognized by the fact that you can’t tell it apart form all the other EFP.
Do not write it, and try not to read it. Read widely outside the genre. Read about the Old West (a fantasy in itself) or Georgian London or how Nelson’s navy was victualled or the history of alchemy or clock-making or the mail coach system. Read with the mindset of a carpenter looking at trees.
Apply logic in places where it wasn’t intended to exist. If assured that the Queen of the Fairies has a necklace made of broken promises, ask yourself what it looks like. If there is magic, where does it come from? Why isn’t everyone using it? What rules will you have to give it to allow some tension in your story? How does society operate? Where does the food come from? You need to know how your world works.
I can’t stress that last point enough. Fantasy works best when you take it seriously (it can also become a lot funnier, but that’s another story). Taking it seriously means that there must be rules. If anything can happen, then there is no real suspense. You are allowed to make pigs fly, but you must take into account the depredations on the local bird life and the need for people in heavily over-flown areas to carry stout umbrellas at all times. Joking aside, that sort of thinking is the motor that has kept the Discworld series moving for twenty-two years.