You can only claim/collect the skulls under the Normal, Heroic or Legendary difficulty settings. 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. Not all skulls earn an achievement; only nine of them do so. To use skulls, press X at the lobby screen. This will give you a bonus multiplier for co-op and go towards earning the Guerrilla, Demon, etc. achievements.

Black Eye Skull
From the start of the "Crow's Nest" mission, without moving anything, you can see a massive pipe above you. Walk up the stairs, then up the next set. Turn to your left and jump up on the shelf unit. From there, jump to the red cross beam then up onto the pipe. From there, just walk to the end. Walk over the Black Eye Skull to claim it. You do not have to pick this skull up, it does it automatically. Its description reads "Bash your way to better health." Your shield will not auto-recharge anymore, and you must kill an enemy with a melee attack.

Blind Skull
In the first mission, get to the area where you see the Phantom overhead. A Marine will point it out. Jump over the rocks and follow the path on the right. When you reach the cliff, there will be a rock over the side. The skull is at the end of the rock. Its description reads "Shoot from the hip". This skull must be activated before you start a Campaign map.

Grunt Birthday Party Skull
Immediately after the first objective of the "Crow's Nest" mission, while en route to the barracks you will fall down to a pipe. Get to the end that pipe. 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 will be at the end of this area. When this skull is active, headshots will cause enemies to explode.

Iron Skull
After freeing Johnson and killing all the Covenant that come in the Phantoms in the "Sierra 117" mission, walk to the Pelican that arrives to pick you up. Face the cargo bay, but instead of hopping 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. Its description reads "Death carries a heavy price...". Instead of only losing 25 points when you die, you will lose 75.

Mythic Skull
Follow the normal path in the "Halo" mission until you reach a cave. Hug 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. Its description reads "Coverage under the Covenant health plan." It gives a 3x score multiplier for all kills.