If someone is looking to apply Game to his life, what would you identify as the most important change/action to take to get started?I have two answers, one physical and one mental. The single most important physical action is to commit to working out. If you're not working out at least three times a week, you will probably not have the self-confidence or the endorphin flow to maintain any significant Game. Not only will you look better and feel better, but you'll be regularly putting yourself in a "I can do this" frame of mind. A man's mood is heavily dependent upon his level of exercise, so don't even think about trying to master Game if you're not improving your physical fitness.
The most important mental action is to force yourself to see women as they actually are rather than through whatever romantic or idealistic or fearful lens you have constructed over time as a result of your interactions with and imaginations of them. This means no more pedestals, no more rationalizations, no more fantasies, no more silently excusing behavior that you would not accept in anyone with whom you don't wish to have sex.
If she's acting like a bitch, she is, at least in part, a bitch. If she's behaving rudely, she is a boor. If she's arguing nonsensically, she's irrational. Accept it and deal with it according to the principles of Game, don't continue to manufacture a host of reasons why it's okay for her to behave in an otherwise unacceptable manner. And if her behavior merits contempt, don't hesitate to show it. To a certain extent, and allowing for a bit of metaphor, it can reasonably said that men create their women.
If you're not in a relationship, it's even more important to see women as they actually are. Even if she's being pleasant to you at the moment, look very carefully at how she treats those from whom she doesn't want or need anything. That is her true persona.