First, the questline must be available in live WoW, all info you see now has to do with PTR.
Second, you must have done a lot of the War Campaign questline. All the way to Nazjatar and getting the quest Circle of Stars, which requires you to defeat Za’qul and Azshara.
Third, one completed, wait until it is available on live servers, which should be this week (1st Oct US, 2nd Oct EU).
Fourth, When the Quest is available on live servers, head to Boralus (Alliance) or Dazar'alor (Horde) and Magni will speak to you and tell you to head to Silithus.
Once completed, you MUST talk to the Earthen Guardian, next to the flight path near the harbor. This will start the questline.
