How do I assemble the mechanical parts of panel mount switch? (lock ring, lock washer, nut, etc)
There are a couple of options.

The "official" way is:


one nut first
locating ring, bent tab towards the panel, to fit in a hole in the panel
panel
lockwasher
second nut. 


Edit: for a "cleaner" front panel, you might put the lock washer on second, before the locating ring.  For thicker panels, various bits may be omitted to make it fit...

Another way:


one nut
locking ring, tab away from panel
lock washer
panel
second nut.


In both cases, the first nut is adjusted so the bushing sticks through the panel far enough to get the second nut on.

The second method depends on the lock washer to prevent rotation - but a toggle switch is not likely to rotate anyway.