The skull is at the top of the large circular room where Cortana says "It was the coin's fault! I wanted to make you strong, keep you safe (drop in tone). I'm sorry, I can't..." When you enter this room, you will fall out of a hole in the ceiling, next to a sitting, dead Elite. To the left of the room will be some small, somewhat fungal, growths on the walls. Near this is a rounded pillar.
Stand on the pillar and jump to the thin growth outcrops to the left. On many of them, you will hit your head on an obstruction above if you are not careful; spoiling the jump. After the first two to the left, turn around and look away from the rock wall - then left ninety degrees for another one with a longish extrusion of Flood material.
This one is difficult as your head will hit the wall with jumps from many of the obvious positions. One jump that works is at the tip of this jump, back about 2 feet from the edge, and jumping slightly left (approx one foot) of the tip of the target jump. Another jump that works is; When you get on the upper outcrop on the left, facing the other outcrop you are jumping to, move as far as you can to the left. Then, jump toward the other outcrop with the extension of flood material. Walk up the extruded Flood tentacle onto the roof of the area, where you will find the Skull in a small pile of blood and bones. Note that underneath the platform you usually walk on are two to three Deployable Covers near the Terminal with Cortana's message. These can be placed on the pillar and the ledge to make the jumps easier and help you avoid things that can cause your jumps to fail.