The Skulls for each level of Campaign mode will appear only once a "kill requirement" is met (example: "Kill 100 Grunts").
I've listed this requirement for each Skull below, as well as the Skull's location. Other than the first mission, all Skulls can be found on Easy. Skulls can be activated from the pause menu and each has a unique effect in game. The effect is also listed below.
1. Alpha Base
Look Daddy Skull - Kill 100 Grunts
Do this one on Heroic or Legendary or there just aren't enough Grunts to kill. The Skull appears across the frozen pond from the entrance to Alpha Base, at the end of the level.
Skull Effect: When Grunts die their methane tank always shoots off
2. Relic Approach
Grunt Birthday Party Skull - Kill 20 Jackal Snipers
These guys appear on pads on mountains, sniper towers and landing pads. Before advancing through the gate to the end of the level, search the remainder of the level for Jackals. There are plenty even on Easy. The Skull appears at Alpha base, by the entrance you took in the last level (south).
Skull Effect: Grunts Explode into confetti.
3. Relic Interior
Cowbell Skull - Kill 45 Hunters
You'll be able to rack up your Hunter count at the very end of the level, where they spawn infinitely from doors just before the exit tot he landing zone. Kill 45, then reverse with everyone to where you met the troops, in the center of the artifact. You'll have to backtrack amidst lots of Grunts and other respawning enemies.
Skull Effect: Destruction physics multiplied.
4. Arcadia City
Wuv Woo Skull - Kill 50 Elites
You'll have to augment your bases with turrets and patrol the streets with hornets to kill the 50 elites (with energy swords). The skull is near the base spot in the east, between your open spot for a base and the enemy base. There's a park southeast of the broken highway and the skull is here -- the furthest southeast corner of the map.
Skull Effect: Scarabs shoot rainbow beams made of pure love.
5. Arcadia Outskirts
Fog Skull - Kill 5 Wraith Tanks
These purple tanks will continue to assault your bases so while you are out exploring, arm your bases with a couple of Rail-augmented turret guns to wipe out any Wraiths that appear. This requirement should come easily -- and hey, you have all the time in the word. Now, follow the strip of highway along the southern part of the level past the southwest base to find the skull. There are plenty of enemies up here, so bring some air units and warthogs and make it quick. The freeway bends away from the two lane strip at the southern end of the map, but you should stick to the two lane. Follow it to the westernmost corner of the level, the skull is here, near the large buildings.
Skull Effect: No minimap. Final score will increase.
6. Dome of Light
Sickness Skull - Kill 50 Banshees
Banshees should be no problem here, even 50 of them. Just outfit your base with Missile turrets and it will pick them off one by one. Leave a Wolverine anti-air unit at each Plasma Rhino position point to help pick off more or the ODSTs will get them all. After you get 50, about face at the base and grab the skull in the garage.
Skull Effect: All players have 50% fewer hitpoints. Final score will increase.
7. Scarab
Rebel Sympathizer Skull - Kill 10 Locusts
Locusts are the equivalent of heavy tanks of the Covenant and walk on four legs. You can find these (there are 10 exactly) roughly nearby the Scarab's 7 nodes, with a few extras on the far east and west sides of the map. Kill these, then search the northwestern-most power node site for the Skull. Knock out some nodes while you hunt Locust or it'll be much more difficult. Gauss Cannon-augmented Warthogs are your best friends.
Skull Effect: Non-player units get 25% more hitpoints. Final score will increase.
8. Anders's Signal
Rebel Supporter Skull - Kill 750 Infection Forms
This kill requirement is pretty high, and achieving it can be tedious. You'll get the most kills out of the little swarms, but any other infection forms count, so long as they have legs. The best way to do this is to go about your business killing everything in your path, and, once you get a base, equip it with incendiary Turrets to rack up the kill count.
Continue patrolling for any alien lifeforms until your count reaches 750. Now, head to the northern-most point of the map in the center, The skull is in a little valley across a chasm.
Skull Effect: Non-player units do 50% more damage. Final score will increase.
9. The Flood
Rebel Leader Skull - Kill 20 Flood Stalks
The Flood Stalks are the bulbous, small stalks that litter the map. They don't fire or do much of anything, they just passively await your Hornet fire. Get a dozen Hornets and explore the map, taking these out -- there are more than 20 available. After you meet the requirement, head back to your base and summon a Pelican drop to take some troops or Warthogs over to the Skull site. It's due east of the base, just a screen away. You can't get to it via ground paths, so the Pelican drop is vital.
Skull Effect: Non-player units start with extra veterancy. Final score will increase.
10. Shield World
Catch Skull - Kill 350 Swarms
Swarms are the flocks of bird-like Flood that will continually assault your base on this level -- but eh, that makes things easy. Equip the base with Missile Turrets and leave some anti-air Wolverines around to make short work of them.
You'll encounter a lot of swarms during your last rush to the evac point, but the Skull should be collected before this point. We noticed a glitch that made it so we couldn't build more Hornets after the final evac was initiated / bungled, and you need Hornets to grab the Skull. It's on an island northeast of the base, just past the first pylon. Bring a few Hornets -- have these built ahead of time and waiting at the base -- to grab the Skull.
Skull Effect: All player units recharge unit abilities in half the time. Final score will decrease.
11. Cleansing
Sugar Cookies Skull - Destroy 100 Sentinels
The Sentinels are the flying bots that help you clean the hull. Destroying these efficiently means Turrets, and lots of em.
Augment them with anti-air missiles too. This level completes when you clear the flood off the ship -- namely the Dens and Launchers. So don't finish those off until you've met your Sentinel quota. After this, head to the very front of the ship, northeast side of the map, where there's an observation deck between the two sides jutting out. Grab the Skull floating in front of the windows.
Skull Effect: All player units have 50% more hitpoints. Final score will decrease.
12. Repairs
Boomstick Skull - Kill 12 Spirit Transports
The Covenant transport ships can only be destroyed when landing, so be sure to build plenty of Missile augmented turrets to catch them early. Repair the outer hull turrets as well, and place a Spartan, Hawk and a Wolverine at all the spots you see them landing. This isn't so hard, as they relentlessly show up. After you kill 12, grab the Skull on the main structure at the front end of the ship, in the northeast. This is hovering above what appears to be the bridge or observation deck at the front of the ship.
Skull Effect: 5% chance any non-player unit blows up upon death with explosive area of effect damage. Final score will decrease.
13. Beachhead
Pain Train Skull - Kill 10 Bomber Forms
All the Bomber Forms, the giant puffy floating Flood forms, are in the far west corner of the map. Here they are being churned out by Flood Dens. Take a pack of Vultures over before conquering the 4th and final Covenant base and clear out 10 of the Bomber Forms before taking out the Dens. The Skull will then appear here in the far west corner at the base of the Forerunner structure with all of the Flood Dens.
Skull Effect: All player units train 50% faster. Final score will decrease.
14. Reactor
Bountiful Harvest Skull - Kill 20 Vampires
These horseshoe shaped purple aerial attack vehicles will show up in pairs to attack your bases and Elephant / Core throughout the level. Wait before taking out the last base at the top of the ramp until you have downed 20. Use those Missile upgrades for your Turrets! The Skull appears behind your third base point, halfway up the tall series of ramps leading to the core drop off point. It's in the northern corner on the platform with the base
Skull Effect: Supplies come in 25% faster. Final score will decrease.
15. Escape
Emperor Skull - Kill 3 Scarabs
Scarabs are the uber units of the Covenant. These giant purple walkers will spawn slowly throughout the level from the Covenant base the northwest of the start point. There should be one already patrolling the shielded center area by the time you have some Vultures in the air, so use them to open both Interlocks of the first set of shields and drop the Scarab into the abyss. This takes a bit of positioning but isn't that difficult. If you set up a base on the direct opposite side from where you start, due north, you should be able to catch any further Scarabs. Fortify this base with Vultures and Railgun Turrets to help. Drop Mac Blasts and Carpet Bombs to make easy work of them. The Skull will appear in the northeast boundary of the map, between two interlocks and behind the Flood boss. Grab it with some Vultures or ground units.
Skull Effect: All player powers recharge in half the time. Final score will decrease.
