You need to jump through the rings (I assume you know what rings, if not, they are the large rings at the end of the level The Covenant that you pass while advancing on Truth with the flood.) in this order:
4-6-5-4-5-3-4
There are seven rings. Number 1 is the ring closest to the entrance, number seven is the ring closest to where Truth dies. You may need to jump through multiple times, Halo just randomly fails and doesn't count it sometimes, even if you do it perfectly. You may also need to wait until every single enemy has been defeated (including all the flood, once they turn against you). When you sucessfully jump through the rings in the correct order and Halo registers it, they will light up. Believe me, when it works, you're gonna know.
Here is a guide (with pictures), in case any further info is needed: http://www.halo3planet.com/halo-3-skull-locations/iwhbyd-skull/
I Would Have Been Your Dad (IWHBYD)
it is very complicated to get the skull, you have to watch videos on how to get the skull
i DONT KNOW FIND OUT YOURSELF
The iwhbyd skull makes Your enemies (the covenant) Use more melee Attacks
iwhbyd adds special dialogue throughout the game
the primary ones are: iron skull, black eye skull, tough luck skull, catch skull, cloud skull, famine skull, thunderstorm skull, tilt skull, mythic skull and the secondary ones are: blind skull, cowbell skull, gruntbirthday party skull, iwhbyd skull and all of them are accessible from the start so you dont have to look for them. hope i helped :P
it unlocks hidden dialogue and pictures throughout the game
You Don't Have To Finnish The Level When You Pick Up The Skull. You Only Have To Finnish The Level "The Convent" When You Get The Skull "IWHBYD" Or " I Would Have Been Your Daddy" To Get This Skull You Need To Go Through The Rings In This Order 4 6 5 4 5 3 4.
Yeahi remember myself trying to get it... Its the IWHBYD skull correct? The ring order is 4654534 and i believe the ring with the red on it is the 4th one so you don't loose track
You're probably jumping through the rings in the correct order, but if you are touching the rings the skull will not appear. It also has to be on Normal or harder difficulty.
I believe you are missing the IWHBYD (I Would Have Been Your Daddy) skull. Pudding 101 answer: You get it from the Covenant level after you kill truth. (You have to jump through the rings in the order 4654534)
the code for the skull hat in dino run is SKULLE