Well, i'll list the name of the skull, the level and were you can find it and any instructions to get the skulls you'll need.

Ok, too start with. You HAVE too play through campaign on normal for the skulls to be there.
Once you find the skull make sure the game is saved, if not you won't have the skull.
And finally when collecting the skulls you may notice some don't give achievements, these silver skulls will still be there in the meta game options so don't worry.

Skull number - 1
Level - Sierra 117
Skull name - Blind Skull
Effect - HUD and weapon graphic are removed
Location in level -
At the first Convenant shuttle drop (Phantom), there is a cliff overlooking a bottomless water pit; check the cliff's edge for a rock outcrop with this skull. Follow this river near the Covenant shuttle drop and go to the edge of the cliff. Without falling off, you should be able to locate a rock outcropping with the skull easily.


Skull number - 2
Level - Sierra 117
Skull Name - Iron Skull
Effect - Dying restarts players at start of the stage, not from checkpoint
Location in level -
Near the train depot where Sgt. Johnson is held. The bridge where the hammer using Brute is, players can jump onto the sides and onto the side of the train depot. Follow the walkway around the big building to get to the skull. The skull is on the upper walkway of the big train shed building in the distance. Get up there using the wall and boxes to the side of the bridge with the hammer and Brute.

Skull number - 3
Level - Crows' nest
Skull name - Blackeye Skull
Effect - Shields recover only from successful melee attack.
Location in level -
In the room where Master Chief starts off (where Commander Keyes delivered her briefing). The large pipe suspended above the room has the skull. Jump to it using the various elements in the room. The skull is on the large brown pipe above the command center. To get to the top simply jump from the red girder up there.

Skull number - 4
Level - Crows' nest
Skull name - Grunt Birthday Skull
Effect - When you headshot a grunt confetti comes from it's head.
Location in level -
After defending the hangar, players are sent to the barracks. The skull is under a hallway with small sewer tunnels and the flying aliens flitting in and out. Before dropping off the maintenance tunnel, players can nudge themselves into the lower room with a well controlled fall. If players fail, immediately restart from the checkpoint, since there is another checkpoint at the bottom of the fall.

Skull number - 5
Level - Tsavo Highway
Skull name - Tough Luck Skull
Effect - Enemies always dodge your shots and grenades
Location in level -
After disabling the anti-vehicle energy field in the tunnel, there will be a large oil pipeline to the left of the main road. Get onto the pipeline and look for one of its supports (it is almost all the way down the canyon drop). Follow the concrete support to the end and there will be a rock outcropping with the skull. Work back towards the road once you are done, and should go under the highway and get back on solid ground. This is the only Alaskan oil pipeline (in Africa) in the game. Locate it and stop the car. Jump on the pipeline, then over the security fence. Look down from the pipeline and you should find this suspicious looking concrete support a ways down the canyon. The skull is on a rocky ledge near the concrete support. Jump for it.

Skull number - 6
Level - The Storm
Skull name - Catch Skull
Effect - Enemy grenades are super-fast.
Location in level -
At the first beach, players need to destroy two anti-air Wraiths (enemy tanks). Ignore the tanks (you can destroy the enemy Ghosts) and alocate a small silo like structure. The skull is on top of this structure. Use the transport Warthog on the beach (or keep the ones you started with) as an extra step to reach the top of the silo. Another player (or deployable cover atop a vehicle) is the best method to add height to one's jump. Note that destroying both Wraiths (by you or by allies) will delete the skull (new map, new checkpoint).

Skull number - 7
Level - Floodgate
Skull name - Fog Skull
Effect - Radar is removed
Location in level -
Within a minute or two from starting the stage, players will encounter the outskirts of a destroyed city. On the right, there is a flaming office building. Atop the roof, there will be a Flood that will jump across the street (roof to roof). Shoot the enemy as he is jumping and the skull will generally be flung off the roof and onto the street (restart if
you miss or if the enemy escapes). Multiple players firing on the enemy helps ease the pressure of any lone gunman.

Skull number - 8
Level - The ark
Skull name - Famine Skull
Effect - Weapons hold less ammunition
Location in level -
After Sgt. Johnson orders players to locate the Map Room "on the other side of this wall", follow his Pelican to another rib structure (similar to the Cartographer's Room). Stop there and use the rocks to get atop the structure. From atop the structure, locate the Covenant communications array on the ground; that array is near the rib where the skull is. To get on top of the rib, simply use the deployable cover as a stepping stone (place it on the slope) and jump up.

Skull number - 9
Level - The ark
Skull name - Cowbell Skull
Effect - Bigger explosions
Location in level -
Head into the Map Room after defeating the Scarab. There will be a large two-level room with a purple energy column right before the room with the skull. The room with the skull has a long ramp (going down) and several cubbyholes stacked vertically at the bottom of the ramp; players need to use the gravity lift to boost their jumps to the top-most cubby where the skull is placed.

Skull number - 10
Level - Covenant
Skull name - Thunderstorm Skull
Effect - All field enemies rank up
Location in level -
After destroying the first anti-air node, Hornets will be dropped so players can find and assist Sgt. Johnson's team on the third anti-air node. Take a brief detour to the second anti-air node (the one the Elites took out) -- it looks like an Egyptian Ibis head set into the cliff. Land on that structure and locate the skull on the tip of the building's "nose".

Skull number - 11
Level - Covenant
Skull name - IWHBYD (I Would Have Been Your Daddy) Skull
Effect - Unlocks new character dialect
Location in level -
After destroying both Scarabs, players will enter the Ark Citadel building. Inside, there are seven rings, separated by three rooms. The Prophet of Truth will be at one end, the Ark's entrance will be at the other. Mentally number the rings in order from the entrance (1, 2, room, 3, 4, 5, room, 6, 7, Prophet of Truth). Meet the Prophet of Truth first, then clear out the enemies back to the entrance (but don't leave just yet). Players must now jump through the following rings in this order:

4 - 6 - 5 - 4 - 5 - 3 - 4

Only one player needs to do this (other players can wait, or jump through one of the rings in the sequence, allowing the first player to skip that ring in the sequence). If a mistake is made during the sequence, it may be restarted from the first set of jumps without penalty. Once the last ring is jumped through, the skull will teleport onto the platform where the "extend bridge" switch was (thus why you need to meet the Prophet first before doing this sequence). On single screen mode, the rings will blink in sequence (as if fired) if the sequence was done correctly. On split screen, the rings will not
blink at all.

Skull number - 12
Level - Cortana
Skull name - Tilt Skull
Effect - Enemies have higher pain thresholds and do not stun easily
Location in level -
It is in the room where Cortana's voice will ask "Do you like playing games?" Look for a purple console (may appear different as the ship is infested) that allows you to jump onto the Flood made out-crops on the side of the room. The skull is atop a large Flood tentacle and the challenge is getting to the top without jumping and bouncing off the sides of the wall. If you cannot reach the Flood made ledges from the purple console, know that Master Chief and Arbiter can stand on the smaller (but higher) protrusion that makes up the
console and use that as a platform.

Skull number - 13
Level - Halo
Skull name - Mythic Skull
Effect - Game's difficulty is boosted by one from the current setting
Location in level -
Upon landing on Halo, follow the right wall to the cave. In the cave, follow the right wall to a dead-end with the skull.

Here is a vid if your still stuck trying to get them

Nice, if i didn't already have them, id use this.