It depends on what kind of women he tends to be able to charm. For a stereotypically "charming" guy, usually he is polite, sincere, and complimentary in a way that comes across as honest--which is what makes him charming. He also is usually smiles a lot and is capable of making people feel at ease because he is comfortable with himself and can avoid saying most things that are outright disturbing or might make people uncomfortable. If he touches a nerve he'll probably notice and apologize in a sincere way, but won't throw himself down on their mercy melodramatically.

Also, most charmers are natural flirts. They don't have to do it consciously, there's just a tinge of innuendo in what they say because they enjoy being playful. And a good way to play a charmer is to make sure when they say something they do it lightly, but with a hint of...inclusion. They want the other person to feel like they can be in on the joke, too, but maybe it's just between the two of them. Something special. You don't have to use a lot of puns or innuendo. Sometimes the inflection and body language of the character is more than enough.

But foremost: playful honesty. Guys that come across as sincere because they mean all the nice things they say, but not so open they're a book to anyone who comes across them is a good way to handle a stereotypical charmer. The hint of mystery can make other characters curious, but they become comfortable around the guy anyway. A lot of charmers come across as really enjoying life, or maybe being involved in The Game of Life that they just might be willing to let other people in on.

I know this isn't concrete specifics, but it's kind of hard to pin down exactly how to act like a charmer since characters can be charming in a variety of ways. I hope it does help though.