Any advice on how best to do this without losing all the washers or going insane trying to get the damn things on both sides of the arm?

Don't do it over a shag rug like I did.

Over a bowl or tray to catch falling parts.

Over a bowl

An empty bowl that is not one full of cheerios:)

and definitly not a toilet bowl

Hahaha, as funny as if Tick would have said it guys !

But yeah, when dealing with x-metal, make sure to keep all the pieces, right Alex ?

actually I wouldn't get too intimidated by switching arms, I actually find it easier than messing with the gromits on the orbitals when I take out my lenses to give them a thorough clean...but then I have stupid, rugby-playing, sausage fingers

here's my tips:

use a simple sewing thread to hold all the washers together..

once you align the arm on the frame
the washers wont be aligned properly
but there should be a little hole where the (unaligned)washers make
use a sewing needle and once u get it thru that little hole, twist the needle around and get the washers aligned
ta-da, screw it in and you're set.

But yeah, when dealing with x-metal, make sure to keep all the pieces, right Alex ?

I KNOW I KNOW! (knocking head on desk in fury)

And the Donkey Award goes to.......ME!