Talk to Surok Magis outside the Tunnel of Chaos, at the statue east of Varrock by the sawmill. If he's not there, click excavate on the statue, then enter and leave.
After the cutscene, follow him into the tunnel
Follow him into the portal in the western room (where Dakh'thoulan Aegis is).
Follow Surok through the Chaos Tunnels portals in the route in the above image.
Be aware that there are aggressive monsters, dragons, and prayer-draining monsters on the way. It is recommended to bring food and combat equipment.
You may be randomly teleported by a portal - if so, locate yourself on the map and make your way back to the route.