How does the whole backstabbing thing work. what do you need to do to get it to work?

Stand behind someone while Hidden in Shadows or Invisible.

Then attack with a select group of weapons: short sword, long sword, katana, scimitar, dagger, etc.. Almost any non-blunt weapons, except for the two-handed and bastard swords, will work.

I have been having problems with the fire giants by the Marching Mountains inside the building. I ahve already placed a forum for that but, can you backstab them?

I doubt it, since you're only about up to their thigh. :D

Seriously though, I don't think you can backstab a non-humanoid. Maybe this applies to Fire Giants.

You can backstab non-humanoids, such as dogs and other things like that. But as with Black Rose, I'm not sure if you can backstab a giant, I *think* you can. If I remember correctly you can, but dont qoute me on that, I think I remember backstabbing the fire giant on level two of watchers keep, but again I cant say if I remember correctly.
Hope that helps.

sure you can backstab fire giants, I do it all the time my f/t usally gets a fire giant from badly injured to near death backstabbing them :). if you have trouble with them use backstab I passed the marching mountains without getting a scratch this way.

A little guide to backstabbing is in here ... http://www.sorcerers.net/Games/BG2/thieves.htm
... the page will be updated soon, with more stunning truth abouth thiefs (and it's better structured then too) :D