Just incase you're too dam lazy to go find them yourself

You can only collect Skulls on the Normal, Heroic, and Legendary difficulty setting. You
must start from the beginning of the mission. You will not be able to see the Skulls if
you load from a checkpoint. In Co-op mode, as soon as one player picks the Skull up, all
players will receive the achievement if there is one. Not all Skulls earn an achievement;
only nine of them do. To use Skulls, press X at the lobby screen. This will give you a
bonus multiplier for Co-op mode and go towards earning the Guerrilla, Demon, etc.
achievements. 

Black Eye Skull: At the start of the "Crow's Nest" mission, without moving anything, you
can see a huge pipe above you. Walk up the stairs, then up the next set. Turn to your left
and jump up on the shelf unit. Jump to the red cross beam, then up onto the pipe. Then,
just walk to the end. Walk over the Black Eye Skull to collect it. You do not have to pick
this Skull up, it does it automatically. The Skull description reads "Bash your way to
better health." When this Skull is active, your shield will instantly charge when you use
melee attacks, and will not auto-recharge anymore. 

Blind Skull: In the "Sierra 117" mission, when you get to the area where you see the
Phantom overhead (one of the marines will point it out), jump over the rocks, and keep
following the path on the right. When you get to the cliff, there will be a rock over the
side. The Skull is on the end of the rock. The Skull description reads "Shoot from the
hip." Note: This Skull has to be activated before you start a campaign map. 

Catch Skull: In "The Storm" mission, go to where the first anti-air Wraith tank is
located. There is a large gray cylinder in the that area on the ground on the far side
from where you enter. The Skull is on the top of the silo. To get up there, perform a
grenade jump off a Warthog or use a vehicle to push a large crate to the side of it; jump
on the crate, then the silo. Note: Do not destroy the Wraith near the door or the Skull
will disappear. The Skull description reads "Pull pin. Count to three. Throw." When this
Skull is active, every NPC will carry two grenades and throws them often. 

Cowbell Skull: In "The Ark" mission, pick up a Grav Lift from the small building near
where you fight the Scarab. Then, proceed through the level until you reach the second
sloping hallway (stairway). You should see some partitioned risers (platforms) halfway
down. The Skull is on the top level. Throw the Grav Lift on the right side of the hall so
it lands on the fourth little green dot from the door. Then, run, jump, and use the Grav
Lift to propel you to the top. You should reach a checkpoint just as you enter the room,
so if you miss you can try again. When this Skull is active, explosive force is increased.


Famine Skull: In "The Ark" mission, when you first go into the valley to the right after
the wrecked Phantom, look to your left to see a huge boulder. Use a Ghost to get to the
side of the boulder that is closest to the bridge overhead. It is easy to pilot the Ghost
up the side of the wall using the thrust. To get the Skull, pilot two Ghosts up the wall
to the top of the bridge and stack them on top of one another next to the beam where the
Skull is placed. Then, jump from the top of the Ghosts towards the Skull and land on the
beam. The Skull description reads "Trust us. Bring a magazine." When this Skull is active,
dropped weapons will have very little ammunition compared to normal. 

Fog Skull: In the "Floodgate" mission, as you are walking down from the anti-air gun you
destroyed in the previous mission, there will be a ramp (next to a missile launcher).
Around this ramp, there is a checkpoint. At this point, you should also hear a marine
yelling, “There! Over There!”. Look up and to the right, directly at the roof of the
building next to the missile launcher. There is a Flood form (not to be mistaken with the
two other Flood forms jumping in front of you), which is holding the Skull. Kill him
before he jumps, and he will drop the Skull down to the ground where you can retrieve it.
Note: Correct timing is required because if you kill him too early, the Skull will get
stuck on the roof. The Skull description reads "You'll miss those eyes in the back of your
head." When this Skull is active, your motion sensor disappears. 

Grunt Birthday Party Skull: In the "Crow's Nest" mission, immediately after the first
objective, while en route to the barracks, you will fall down to a pipe. Go to the end of
that pipe, then look over the edge to find a small space a few feet below. Drop over, then
quickly pull back to land under the floor you dropped from. The Skull is at the end of
this area. When this Skull is active, it makes Grunts explode in confetti when they are
headshot. There will also be some cheering, like a birthday party. 

Iron Skull: In the "Sierra 117" mission, after freeing Johnson and killing all the
Covenant that come in the Phantoms, walk to the Pelican that arrives to pick you up. Face
the cargo bay, but instead of going in, turn left and follow the path. Once you hit the
wall, jump up to your right to eventually reach a walkway. Walk all the way to the end.
Take a left, then a right. At the end of this, a little bit to the right is a dark corner.
Press Up to turn your light on and you should see the Skull. If you cannot see it, you are
probably standing on it. The Skull description reads "Death carries a heavy price...".
When this Skull is active, instead of losing 25 points when you die, you will lose 75. 

IWHBYD Skull: In "The Covenant" mission, go to the last tower. Before you get to the
Prophet of Truth, there are a series of bridges with Halos. Do not jump through any yet.
First, kill the Prophet of Truth, and defeat the bridges of enemies. Then, jump through
the Halos in this order, with "1" being at the entrance and "7" being the end near where
the Prophet of Truth was located: 4, 5, 6, 5, 7, 6, 5, 4, 1, 2, 3, 2, 7, 2, 1, 3, 4, 6, 7,
5, 4, 6, 5, 4, 5, 3. Alternately, jump through the Halos in the following order: 1, 3, 4,
6, 7, 5, 4, 6, 5, 4, 5, 3, 4. This is much easier in Co-op mode with three or four people.
This is the Halo theme if it was played on a phone by dialing the numbers. If done
correctly, the Halos will start flashing, and the Skull will appear near the Prophet's
ramp. If the Halos do not flash, try again. The Skull description reads "But the dog beat
me over the fence." When this Skull is active, there is bonus dialogue throughout the
game. IWHBYD stands for "I Would Have Been Your Daddy", which is a level chapter from the
original Halo. -From: crimsontwist 

Mythic Skull: In the "Halo" mission, follow the normal path until you reach a cave. Follow
the right wall until it branches off into a small path. At the end of this path is some
rubble, and in front is the Skull. The Skull description reads "Coverage under the
Covenant health plan." When this Skull is active, you will get a 3x score multiplier for
all kills. 

Thunderstorm Skull: In "The Covenant" mission, after you shut down the first tower and get
access to the Hornet, fly to the second tower (the one the Arbiter shut down). While
walking up the stairs, go to the middle part that connects both, and there should be
stairs leading up to a platform where the Skull is. The Skull description reads "Field
promotions for everyone!" When this Skull is active, enemies will be much stronger. 

Tilt Skull: In the "Cortana" mission, when in the circular type room with all the Flood,
look for a small structure piece next to two archways. Jump on top if it, then go up on
the rocks to the top left. Turn left and jump up again, then turn 180 degrees and jump to
the rocks across from you. Then, follow the rock sticking out and leading up on top of the
original circular room. The Skull is in a pool of blood. The Skull description reads "What
was once resistant is now immunity." When this Skull is active, enemies will have
different body parts that are resistance to certain bullet types. 

Tough Luck Skull: In the "Tsavo Highway" mission, once you get past the part where you are
forced to leave your vehicle, you will go through a tunnel and onto an open freeway.
Immediately after coming out of the tunnel, you will see a huge pipe with two yellow
ladders. Climb the second ladder. You should see a Regenerator power-up. Proceed to your
right onto a small platform ledge where the yellow chain link fence is located. Jump onto
that ledge, crossing over the fence. Directly behind you should be a large column that is
holding up the giant pipe you were just on. Below that is a walkway. Fall off the ledge
onto the walkway. Walk to the end of that. You should see a hidden ledge with the Skull on
it. A few attempts may be requires to jump across. The Skull description reads "Your foes
always make every saving throw."