The way I did it on my X400i was this: I carefully disconnected the turn signal plugs from each other, and then screwed the mirror stalk into the handlebar socket. Then, I tightened down the locking nut with a 17mm open-end wrench to the point that it was snug, but not overtight. Then I reconnected the plugs. There is also another locking nut at the top of the stem where the stalk goes into the mirror housing. That one is hell the ruggedest mile to get adjusted, because it takes two hands to hold the mirror housing from moving while you're snugging up the nut. I only have two hands, so I enlisted my lovely spouse to hold the housing while I did the locking nut.
