go back to the bar in castle town, talk to the barkeep or go to the table to the far end of the entry door,, look on the map and it will tell you where you need to go (taht is the map on the table) then from there you should tele to lake hyrule and go to the howling stone there.. if you dont know where that is im not going to explain it cause its to much to explian...from there look in first person and look around till you see a long tower with a ladder you can climb, at the top is the person that the barkeep was talking aboout he'll give you a letter to give to the cannonier and hell sett the destination to garudo desert....