Here is a video that shows all of the locations -
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The trickiest one would probably be on the level Cortana, that level could be a bit confusing (for me anyways) since everything looks the same. Remember to play on Normal, Heroic, or Legendary because the skulls will not pop up on easy mode.

Try to find them by yourself first and if you still need some help feel free to contact me on TTG and I'll have no problem helping you out tomorrow.

Speaking on the level Cortana, I'll tell you something cool about it. There is another skull on the map. If you grab this skull and complete the level with it, you will start the final mission with a spartan laser. I always thought it was a cool thing to do.