Here's a very good explanation from Halopedia:
Halo 3: IWHBYD (I Would Have Been Your Daddy) Skull
In Halo 3, the IWHBYD (I Would Have Been Your Daddy) Skull is located on the level The Covenant. To get this skull, you must start at the beginning of the mission, and be on the difficulty level of Normal, Heroic, or Legendary.
The name of the skull is based on a statement uttered by Sergeant Johnson in the first Halo: "I would have been your daddy, but a dog beat me over the fence!" It is also the name of the first chapter of the campaign level Assault on the Control Room in Halo: Combat Evolved. In order to get the IWHBYD skull, you must complete the entire mission until you reach the final battle scene, where the Prophet of Truth is planning on lighting all seven rings. Once you reach the bridge, battle your way across with the Arbiter and Flood, neutralize all Covenant, watch/skip the cinematic, and kill all the Flood (this is optional, but makes the procedure much easier). Once the bridge is devoid of enemies, you will have to literally jump through the 7 rings in a complex order. Once you have completed this, the IWHBYD skull will appear at the beginning of the light bridge leading to Truth's podium. MAKE SURE YOU DO NOT TOUCH THE RINGS OR JUMP THROUGH THEM BEFORE YOU KILL TRUTH.
Each number corresponds to the ring you must jump through. The ring at the elevator is 1. The ring next to Truth is 7. The ring with red on it is 4.
Sequence order: 4, 6, 5, 4, 5, 3, 4. This is another 7 reference.
Once you have completed the sequence, the skull will appear at the beginning of the light bridge leading to Truth's podium. If you are not sure you got it then look at the rings when you complete the sequence, they will be blinking. The Halo theme will play as you get the skull. You must then finish the level, or the skull will not be saved to your profile and you will have to get it again.
Each of the holographic rings sound a different tone of the E Dorian scale when you stand near it. The sequence, if you pay attention, plays part of the Halo theme song. Also, the rings in the citadel are holographic projections of the 7 Halo installations, the fourth being the newly reconstructed (and unfinished) Installation 04, the yet-to-build areas indicated by the red-glowing sections.