This is from Halo3planet.com
IMPORTANT! START FROM THE BEGINNING OF THE MISSION, ON NORMAL, HEROIC, OR LEGENDARY IN ORDER TO GET ANY SKULL!
The Skull is located in the level, “The Covenant”. After defeating the Scarabs, and going inside, get to the location with all 7 Ring Holograms. You must jump through the rings in a specific order. That order is as follows (1 is at the entrance, 7 is at the end):
4 6 5 4 5 3 4
After jumping through them in this sequence, you will be awarded the “I Would Have Been Your Daddy Skull”. It will be right next to the bridge that brings you over to the Prophet of Truth.
hope this helps buddy. Id get online and help you do it cause ive done it with like 4 friends but my GF wont let me play halo this weekend.