how do I get the IWHBYD skull in Halo 3?!?

[xshutter_bugx]

I am playing the game on normal, I played the mission all the way through. Tried and tried nothing! I didn't realize I had to kill Truth first before jumping through the rings, did that...killed the flood, jumped the rings. Nothing!!! I am getting so frustrated with it! what am I doing wrong?!?
got it! xP

no achievement?

 

[War~Kannon]

After defeating the Prophet and Flood in the level "The Covenant", jump through the halo rings in this order.

1, 3, 4, 6, 7, 5, 4, 6, 5, 4, 5, 3, 4

The IWHBYD skull will appear by the staircase near where the prophet was.