As a former smoker, I can tell you that I am very glad that you are faking it, cause smoking is nasty and gross and smelly and it was a pain in the rear to quit, and BLEEEAACCCHHHH.
That being said, the mannerisms vary from person to person, character to character. Is your character jaded? Posh? Girly? Tough? Why is she smoking in the first place? Answer these questions (and dozens more) first, to give yourself as much information about the character as you can. Do this before you even approach her mannerisms, because once you know a lot about her, her mannerisms will come more naturally.
The time frame of the story, when it takes place, is important as well. If the story is set in the 1920s or 30s, studying old film noirs is a good idea, but if the play is set in the 1970s, study films from that era. If the play is set in the 1990s, watch films from that era, and so on. There is a distinct difference.
And finally, once you have done all your homework, and studied and so on and know what your character's mannerisms are: do them, *repeatedly.* Smokers are completely unconscious of all those little ticks and tremors. They don't realize they are twirling the cigarette, or flicking it, or even bringing it to their mouth. They don't think about how they pack the box or pull out the smoke, they don't generally even look at it, not even when they light it. Some people pack, open, remove and light all in one fluid motion with one hand. Some people use both.
The only kind of smoker (that I know of, or ever had experiences with) that looked at their cigarette, *really* looked, as if contemplating its very existence, was a smoker that still felt surreptitiously guilty about the act of smoking (or was considering quitting and was in the process of developing shame for the act). People who have been smoking for a *very* long time (and these people tend to be pretty cynical about life and the world in general, not sure why it correlates) exhale through their nose. Which is a wholly unpleasant experience unless you have been smoking for a long, *long* time.
I really hope all this helps!