The thing about success, is that it's self-reinforcing, especially in the beginning, right until you reach your plateau and stall. And so what you need is focus on having little bits of success and then ride the confidence wave you get from it from there. E.g. you could make your first goal as just having a regular girlfriend in a relationship, and then perhaps stray while in it. This is one of the easiest ways to be a fuckboy because getting it on with some other chick is easier when you're in a relationship. And the confidence rush you get from it will push you further and further in your fuckboyry. Though you'd need to mind your girlfriend's feelings, and if she ends up hurt by it, try to compensate the hurt and make it up for her somehow (do her some romantic gesture or take her on a trip or something).