The requirements for the ghostly robes is to just have Desert Treasure started and have attained the ring of visibility... Once you have that, you'll also require a ghost speak ammy (refer to the Restless Ghost quest if you have lost it) ... Goto glarials tomb (near the waterfall), and you'll see a ghost wandering around (make sure you have ring and ammy on), that's how you start the mini-quest.
Good luck
It is a minigame called ghostly robes information can be found at tip.it but you need to have completed a part of DT and you must have a ghostspeakk ammy.