Alright so this skull can be a little painful if you fail to get it as you have to wait quite a lot to set it up if you miss it.
First you have to be on the mission "The Armory" in halo 2. At the start of the mission when Johnson comes down in the elevator you must wait outside Untill he says "would it help if I said please". Once he says that walk inside and you'll be taken up to the tram & once you reach it step inside.
Now this part is the most difficult part if you don't do it right! As you are nearing the end you'll see marines on the platform and a skull sitting on the chest. Now you want to push towards the window and push
and hold it Untill you pick up the skull, but if you fail you can always go to last checkpoint and repeat these steps again :)