How to reach Old Sebilis
The best way for you to reach Old Sebilis is by taking the PoK stone to Cabilis and traveling northeast to EJ (Emerald Jungle). If you're a Wizard or Druid you'll be able to port directly to EJ and skip this initial part. Once you're in EJ you'll want to head southeast to the Trakanon's Teeth zone line, shown on my map below.
Zone to TT and get ready for the super long annoying run... From the EJ Zone line you're going to be heading all the way southeast - through the ruined city area and down into a passage way which will lead you to Old Sebilis.
If it's your first time here the passageway may be a bit difficult to find, just zoom in on the map and you'll see where you have to go.