First, head to the training dummies in the Valley of Honor and go into the building to your left.
Then go into the south western side of the building then jump up on the wall.
Head to the other side and climb up the wooden pole, make sure your at the very top, walk slowly and dismount so you don't fall off.
Now, fall onto the building in the picture (this may take a few attempts, don't jump, you have to just run towards the building.).
Run around the roof to the wooden fence, now jump over it and you'll land under Orgrimmar (be careful there's a gap between the fence and the roof).
Enjoy.