Halo 3 Skull Locations - Gold Skulls
You can find the Iron Skull at the tail end of Sierra 117, the very first level. It waits for you behind the building that Johnson is being held in, up on the rooftop. The far right side of the building will give you access to the roof, where you'll find the skull on the left side, hidden in a corner.
Black Eye Skull
This skull is located in the room you start in on the second level, Crow's Nest. Head up the higher level in the room, towards the door with the red light, and then turn around. From there, jump onto the racks, onto the red metal light holders, and onto a ventilation pipe from there. The skull should be on that pipe.
Tough Luck Skull
This skull is on the third level. When the Covenant cruiser flies overhead, look to your left. You should see some large piping running parallel to the road. Underneat the pipes are support beams that extend over a cliff. Jump onto them and follow them out. Once you hit the end, look to your left and you should see the skull on a rock.
This skull is on the fourth level in the first area where you battle the Wraith tanks. Do not destroy them. If they go, the skull goes. Instead, head out to the circular building next to the anti-air tank. The skull is on the roof, which you can get to by either grenade jumping or jumping off of a Warthog.
Out of all of the Halo 3 Skull Locations, this one is the easiest to find but a little tricky to get. It's located at the very beginning of the level. As you're walking away from the destroyed anti-air gun and towards the level, you'll hear a marine shouting. If you look up, you'll see a Flood creature hopping across the rooftops. Kill it and it will drop the skull. It takes some quick and deadly aim to hit it. Also, if you shoot it too early or too late, the skull will be stuck on the roof.
This is another one that's tricky to get, but I think it's a great deal more frustrating than the Fog Skull. You can find this one in the sixth level. At the point in the level where the path forks, take the right path. You'll notice a structure above you. You need to get up onto that structure. There are multiple methods of doing this and all involve either vehicles or grenade jumping. Some people claim that Ghosts and Brute Choppers can drive right up the rocks to the right of the structure. The best advice I can give is toy with it until you get it.
Once you're up on the structure, head down to the last support beam and look out at the end of it. You should spot the skull. From there, your best bet is to grenade hop over to it.