Someone please help! I usually give answers, but now I'm stuck!!! ..

I don't know but all I know is that on most stages it has to be on legendary to get the skull.

Look in the forums.

It has a list with all the locations and sometimes the effect off all the skulls.

I only know there's one on the level outskirts.

Arbiter: Grunts Birthday Party mode:
Play the mission under the legendary difficulty setting. Proceed until you reach the first Banshee section. Immediately fly straight down to the bottom of the level. Search the cracks where the large cylinders connect to the station to find a circle of Grunts dancing around a skull. Get the skull and the words "Grunt Birthday Party" will appear. When you get the skull, all headshots that kill the enemies will cause them to explode as if a plasma grenade were attached to them. This works well if you aim for a Grunt in the middle of a group of enemies. Also, this may only work for projectile weapons, but does work for the Covenent carbine and the Covenent sniper rifle. Armory: Skull:
Complete the orders until someone asks you to go in the elevator. Wait until he says "Will you come in if I say please?" and walk in. When you get into the tram, face the Earth and hug the wall. Hold X for the entire ride. Eventually you will pick up a skull that says no clue to what it does (no word will appear, as with the other hidden skulls). Note: You cannot keep it as a weapon, as it is taken away after the intermission sequence. However, you can beat up the man with it. Cairo Station: Thunderstorm Skull:
Play the level under the legendary difficulty setting. Play through the level until you reach the second room where the Covenant are boarding. The game will say "Priority Shift". Go through the Armory and take out the Elites. Then, go to the "Commons B.01" and clear out all enemies on the ground floor and the Grunts on the far side manning the plasma turrets. Once the room is clear, look into space. You should see the dual tram tracks. Note that the supports in the center are close together. One of those diagonal supports goes down the side of the wall where the glass railing is locaed. Jump on the post for the glass railing and jump onto the diagonal support. From there, jump onto the tram track and follow it all the way down towards the door you originally came in. You should see a landing with a trash can. The skull is in this trash can. Walk up to the trash can and hold X to pick up this skull. The word "Thunderstorm" will appear. This skull causes all Covenant enemies to be Spec Ops. Delta Halo: Envy mode:
Play the mission under the legendary difficulty setting. Continue until reaching the area during the "push through the covenant held ruins" phase where a landing zone for the Pelican to drop weapons must be cleared. When you reach the area where Cortana says "They're pouring out of the center building", note the plasma turrets along the borders. A short distance after the turret on the left side is the outline of a sealed door in a wall. There is a rock in front of the door. Jump on the rock, then jump onto the ledge above the door. Next, jump to the ledge to the left. Then, grenade jump onto the building with the sealed door. On the roof of the building are two dancing Elites and a skull. Get the skull and the word "Envy" will be displayed. You can now use active camouflage. Gravemind: Angry Skull:
Play the level under the legendary difficulty setting. Fight your way through the Covenant city until you are outdoors. As soon as you are outside, turn right. You will see some Covenant crates next to a rock ledge. Jump from the crate to the ledge. Walk up the hill, and to your right will be a red girder. Jump on the girder and follow it back towards the area you just came from. There will be a cloaked Grunt with a fuel rod cannon. Kill him and continue down the girder. You will see a skull floating above a small gravity lift. If you get this skull, the word "Angry" will appear. This will make Grunts shoot faster, but you can use it as a blunt weapon. The Angry skull is the only skull you can cycle around as a third weapon. The Great Journey: Black Eye Skull:
Play the level under the legendary difficulty setting. Eventually you have to escort the Sarge who is in a Scarab. Follow him along until you see the huge compound in front of you. Directly above the door that the Sarge blows open with the Scarab is an angled beam. Land your Banshee on this beam and walk to the top of it. Look down and to your left to see a circular angled wall. Jump onto this wall and walk to the top. Walk straight along the right-hand side of this area. You will have to jump over a small beam in front of you. Continue walking straight until you come to a grassy area. Keep walking up the grassy hill and go until you reach the point where can jump onto the angled support where the tower meets the grass. Jump onto the support and walk up the support. It is a very steep climb; jumping while you are going up the support speeds it up somewhat. At the first landing, you will see the Black Eye skull sitting on a glowing white panel on the floor. This skull makes your shield charge abnormally. To charge your shields you must melee enemies. You can charge your shields into the overcharge range by doing this. High Charity: Iron mode:
Under the legendary difficulty setting, after you cross the light bridges and fight through to the end of the level, you reach a room where Cortana says "Brood and Elite ships are engaging all around. The next lift will take you up to the conduit. Hurry". Go to the room, enter the lift, and look up while going up. Hold X so that you can grab the skull on the way up. The word "Iron" will appear when you get it. Metropolis: Catch Skull:
Play the level under the legendary difficulty setting. Immediately after you exit from the underground highway, you will reach a large open-air circular room with a handful of Jackal snipers and some Elites. The quickest way to get safely to the skull is to get the next checkpoint by jumping into the Warthog and letting the Marine drive you to the next area. Jump out of it, backtrack to the large room, and clear the area. You will see a large semi-circular beam going from one of the main platforms that goes from the ground up to the top of the two towers. Do a grenade jump at the bottom of this beam and walk all the way to the top. Jump over to the ledge of the left tower. Walk to the end of the ledge and you will find the Catch skull. This skull allows enemies to drop two plasma grenades, and all enemies will toss more grenades in general. Oracle: Famine mode:
After you go down the swirling elevator and enter the room with the big glass windows, destroy all the enemies. Then, grenade jump to the opposite side of the second floor. There will be four Flood soldiers shaking on the ground with a skull between them. Get the skull and the word "Famine" will appear. Outskirts: Blind Skull:
When you begin the mission, your vision will get blurry. When it returns to normal, you will see a blinking door. Enter it, then stop. Look up to see a fluorescent light over the door you just used, and another one on the opposite wall. Jump and duck at the same time to get on one of thos lights. Do the same thing again to get onto the ledge next to the lights. Look to the left on the ledge and you should find a dark passage. Follow it to find some grenades, a skull, and a camera at the end. Take the skull and your HUD will disappear and the word "Blind" will be displayed. You will not be able to see your weapon, body, shields, ammo, or HUD. You can use the skull as a melee weapon, which does good damage to opponents. However, using a grenade, firing a weapon, trying to drive, or using a turret will drop the skull. However, you can be a passenger in a Warthog and still keep it. To return to normal vision, save and quit the game, select the Cairo Station mission in campaign mode, save and quit the game, then return to the Outskirts level. Outskirts: Your Daddy's Skull:
Note: This may require several attempts. Play the level under the legendary difficulty setting. The main thing appears to be getting the checkpoint just past the Pelican that picks up Johnson. This checkpoint spawns the sniper Jackals in "sniper alley". The skull itself is in a room on a balcony. The room contains a sniper rifle and sniper ammo; it will also contain the skull. Picking up the skull teleports you to the ground level and gives you just a Plasma Pistol. You will quickly be assaulted by Elites. Killing the last Elite will have the text for the skull flash. You have to kill all of the Elites to actually finish getting the skull. If you die to the Elites, and your checkpoint no longer spawns the skull, kill yourself four or five times to get pushed back one extra checkpoint. The skull should spawn for you again, and you can try the fight again. When you get the skull and kill all the Elites, the phrase "I Would Have Been Your Daddy" will appear.
Quarantine Zone: Sputnik mode:
Play Quarantine Zone under the legendary difficulty setting. At the start, turn left immediately. You should see an Elite, and just slightly to the left of him should be a cave. When you reach the exit to the cave, there should be a cliff. Follow the cliff to a wall. Along the wall should be a small walkway. Go on the walkway. At the end of the walkway should be a skull. The world "Sputnik" will appear when it is picked up. Explosion and melee forces are greater in this mode. Regret: Assassins mode:
Play the mission under the legendary difficulty setting. Enter the structure after getting off the gondola. Jump on the large stone block to reach a ledge leading out to the Covenant turrets. Turn around and face towards the structure. Then, jump up to another level around the right side of the structure. Continue to find a skull on a ledge as you make it around. Take the skull and the word "Assassins" will be displayed. All enemies in the game are permanently cloaked. Sacred Icon: Mythic Skull:
Play the level under the legendary difficulty setting. There is a semi-outdoor area which has the same type of area directly next to it. There is a large Sentinel in front of each one. Go past this area and down the piston. You will be in a vary dark and dank Flood infested area. There is a stack of six or seven boxes as you enter the room. Use them to jump up to the landing above. There are a handful of alcoves on the right-hand side. The skull is in the third alcove, floating above a dead human Flood. Enemies will have more health after this skull is collected. Uprising: Ghost Skull:
Play the level under the legendary difficulty setting. After the outdoor area with all the Ghosts and Grunts, there is an "L" shaped room that is seen throughout the level. It is immediately after the armory room that is full of Covenant and Human weapons. Grenade jump using a jacked Ghost up to the rafters. The skull is up there. You can also push a box from the armory room into the hallway, and jump on this box. The skull is directly above the door. If you hold X while jumping close enough to the skull, you will pick it up, even if you are not in the rafters and the skull is not visible. Enemies should take longer to notice you after the skull is collected.