First. go to Winterspring, and over find the NPC on the map, and work from there to find the loc of this picture:
Then, mount up and do as the pic tell you
Continue:
and your done!
This is a very good trick to scip the cave in the onyxia prequest.
if you dont got his way, you will find yourselfs killing alot of lvl 56 dragonkin elites, wich you migth need help to kill.
Edit: Great if you dont know how to Wallclimb