I find the least annoying/least stressful thing for them is holding them gently but firmly on my lap, butt end facing toward my hand holding little but good scissors. I gently snip snip off the yukky dried stuff and if bits of feather go too, it is irrelevant because they have a beautiful bum when I'm done and you can't even tell there was any feather snipping. Always seems I have a happy hen after I do this, as if the stuff was bugging them anyway. And once they know the drill, they seem to welcome it.
Takes under a minute - just have to be VERY careful with the scissors, hence why little ones are best.
JJ
p.s. I usually do this before taking my shower!