Location: Progress through the level and you’ll eventually come to a large area that has a number of structures supported by pillars. This area is reached shortly after you’ve extended the bridge, and just before the start of the “You Break It, You Buy It” chapter. If you see the chapter start text, return to the previous area.
When you reach this area, head up the two ramps along the left side. Make your way to the very end of the upper ramp and jump onto the stone block there. From here, jump onto the ridge above. While standing on the ridge, crouch-jump to the platform above. To perform a crouch-jump, jump normally and click the Left Thumbstick to crouch just as Master Chief reaches the apex of the jump to push up a bit higher.
Equip a Plasma Grenade, stand below the wall here, and throw the grenade at Master Chief’s feet. Wait a moment or two and then jump up toward the ledge. If done correctly, the explosion should just push you up onto the ledge. The skull is on the ground between the two dancing elites.
With this skull active, Master Chief can use the camo ability that The Arbiter has. This effect remains active until you turn off your Xbox and reload the game. To re-activate this effect afterwards, you’ll have to collect the skull again.