Instructions
Alliance
Go to Theramore Isle in Dustwallow Marsh. Board the boat to the Menethil Harbor in the Wetlands. Once you reach the harbor, take the only road out of the town and follow all the way until you see another path cut off to the left. Take that path into the Arathi Highlands.
Follow the northern road in Arathi and turn left when you come to the first fork. A few paces ahead, you'll see another fork. Turn right and continue on the road and through a tunnel to enter the Hinterlands.
Follow the road out of the tunnel until the road splits to the right and left. Take the left road until you see a beaten path heading to the right. The path is not as noticeable as the main roads. Keep on the path until you arrive in the Western Plaguelands.
Jump into the body of water that is directly ahead of you once you reach the Western Plaguelands. Swim to the shore on the other side and run straight until you find yourself on a road. Take the road east to the Eastern Plaguelands.
Continue on the road until you reach the Northpass Tower. The road contains several side paths you can turn on to, but just keep on the same path. The Northpass Tower is considerably deep into the Eastern Plaguelands and will take you several minutes to reach. Head northeast once you reach the tower and you'll see a gate. Go through the gate to enter the Ghostlands.
Horde
Board a Zeppelin from Orgrimmar to Brill. Once you land in Brill, take the southern road. Once the road forks, turn left on to another path.
Follow that path through both the Western Plaguelands and Eastern Plaguelands. Stop once you reach the Northpass Tower.
Run northeast and you'll join together with another road. That road leads into the Ghostlands.