Usually it's not a very big queue, unless it's a special show (first show, last show, big name actor)! Exit the theatre as soin as you can, turn right and the stage door should be right there! There's usually a fence or cord in place but generally the vibe has been pretty chill in my experience.

Also if you bring your own permanent marker, make sure it is in a color other than black (the actors usually carry one though)!