I'm 99% sure this isnt a repost but if it is I apologize.
First of all you need to get the Mist-Filled Spirit Lantern from Evermaw, don't ask me why, it just doesn't work without it. (Mist-Filled Spirit Lantern - Item - World of Warcraft)
All you need to do is fly just off the island into the fatigue area, mount up on a flying mount then accept a cross realm zone group invite as you fly back onto the island. If you look at your buffs your mount icon will not be there but you will still be flying around on your mount. The mount is only aesthetic, so basically you can still cast abilities while flying around! This bug lasts until you log out or change continent, you can fly all over pandaria with this however it does not work in raids or other continents. Also if you get slowed (dazed by a mob, slowed by a player, or even if you switch to aspect of the pack or anything else that changes your movement speed) it will make you fly at walking speed and you'll have to log out and get another cross realm zone invite. Also entering the caves can screw it up but not always.
While you are flying it counts you as moving so you can't cast spells unless your standing on the floor, however you can still cast instant casts and hunters + melee can cast in the air. And while you're on the floor you run at ground mount speed, not flying mount speed.
Just a small warning, everyone who sees you will see you swimming in the air without a mount so you may get reported but I don't know whether blizzard will actually take action.
Have fun with this bug, it really is amazing for farming rares or even ganking if you can kill them without getting slowed!