First of all, welcome to Gamebanshee.
Backstab is a passive skill, but to use is with highest efficiency, you must attack a target that doesn't consider you as a threat from the back.
Just initiate a dialogue with him and use the options "I'd like you to do something for me" -> "Wait here for a while".
You just need to use the lockpick set, the easiest way is to drag it in one of the hotkey slots.
Not as far as I know, but you should pay attention to pickpocket the target from the back, that will decrease your chances of getting caught.
As for other hints, I can't think of any more at the moment (probably because I just woke up
), so you might try to browse through some of the threads that you think might prove useful.
