This is most easily accomplished on the first mission: Osiris. Move across the level normally until you reach the point where you need to smash through the ice. The next portion is where the first and easiest skull in the game is located. Run through this open area where there's tons of enemies or kill them if needed on at least Normal difficulty. The skull will be to the left of the exit to the next part of the level and you'll need to climb on a mini cliff side. On the portion of the mini cliff side furthest towards the open outside of map battle you'll find the tiny IWHBYD skull sitting there. Video for assistance below and feel free to check out my full skull guide.