I agree with JTDman, its not an easy job if you haven't got the right tools.
If you buy the whole arm, at least you'll know the ONLY troubles you'll face are getting the old arms off - they can be a pig!
I'd do both sides at the same time if I were you because the one that has given up will have exerted a lot of extra wear on the other side and might also need changeing very soon.
George