If the Garret turbos are anything like the Holsets..... there will be a snap ring of sorts holding in the silencer baffle/ring. I just took a small screwdriver to the ring on the Dodge/Cummins I had and it just popped right out.... ring followed right with it.
I dont know the specifics on the Powerstrokes, just the Cummins. sorry I couldnt be of more help.
and I dont have a manual to read either.