In Halo 3, the IWHBYD (I Would Have Been Your Daddy) Skull is located on the level The Covenant (Level). For this skull, you may be on difficulty levels: Normal, Heroic, and Legendary to acquire the skull.
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 the cinematic, and kill all the flood. 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 on Truth's podium.
There are two sequences that both work. One contains 18 jumps, while the other has only 7. Both will produce the IWHBYD skull, the difference between them is that one take longer. The purpose of the longer code is to provide a "backup" in case you might make a mistake or perhaps the shorter simply doesn't work.
Note: Each number corresponds to the ring you must jump through. The ring at the elevator is 1. The ring next to Truth is 7.
Sequence order:
>Shorter order: 4,6,5,4,5,3,4
>Longer order: 1,3,4,6,7,5,4,6,5,4,5,3,4,5,4,5,3,4
Once you have completed either sequence, the skull will appear at the start at the bridge at the end. The Halo theme will play as you get the skull.
The IWHBYD Skull unlocks the extra dialog in the game, which usually comes from the UNSC Marines and Elites.