Halo 5: Guardians

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.

Of the 13 skulls the first one is probably the easiest as it only requires getting through 2 battles. This is the IWHBYD skull, oddly enough, which is normally one of the more convoluted and difficult skulls to obtain.

The video below contains all of the skulls, but skip to (0:19) for the one I mentioned.

After you get to the clearing where you fight the forerunner soldiers for the first time at the other end of this canyon, you will be prompted to enter inside through some large doors. Simply head to your left and jump up onto the ledge and follow it around. You will see a skull sitting on the edge of the cliff, waiting to be picked up. Pick it up and the achievement is yours.

Video guide: