Talk to Hartwin in the northwest tower of Varrock Palace and he will teleport you right next to the trapdoor leading into the dungeon. Although it is nominally located at the Chaos Temple in the Wilderness, there is no way to access the dungeon from the true Wilderness, nor any way to return to the Wilderness from the safe area. If you walk to the edge of the safe area, you will be given the option of returning to Varrock Palace.