Once you drive out of the forests area, Gibson Steps will come up pretty soon on your left, so don't speed through or you will miss it. Look for the signs.
The main rocks will be after this location (less than 3 minutes drive away). You will see many parking lots on the right side and a building that houses some exhibits and contain toilets.
Your final destination for the 12 Apostles will be after Port Campbell, a small town about 10km after the main rocks. Drive through the town, 5 minutes later you will hit your final destination.
All attractions have signs that are brown in colour.
Best to travel with all photographers in the group, if not you will see that everyone comes back with a BLACK face after the trip. Non-photographers just don't understand, especially girls.
Also, PLEASE DO NOT SPEED along the Great Ocean Road. Or you may end up Sky-Diving into the ocean. These stretch of the Great Ocean Road is very narrow, has no railings, once off and you are gone. There are too many steep bends that requires you to have speeds of between 25km/h ~ 45 km/h every 10 to 20 seconds. Since you are driving an auto car, just drive carefully. Of course, as I'm quite familiar with aussie roads and that I drove a manual car, it is pretty damn exhilariting negotiating the mountains and bends, you get that adrenalin rush!
People nowadays only know how to drive auto, manual is so much fun!
Do not take the Great Ocean Road at night, too dangerous. Take the Princess Highway and cut down using small roads to Port Campbell if you are travelling from Melbourne and vice versa if you must travel at night. These roads have no steep bends, pretty straight throughout. In fact, if you are in a hurry to get to the 12 Apostles fast, it only takes 2.5 (fastest I tried) to get there as compared to 4 hours if you take the Great Ocean Road. Of course, you will miss out on the breath taking scenery of the Great Ocean Road. But at night who cares, you don't see anything anyway.
Forget about The Gramphians, visit only if you have time. I don't find it worthwhile going there.