How do you get a girl who has a boyfriend breakup with him to get with you?

There's a simple rule for this that applies in most cases: don't.

If they're in a happy relationship that they want to last, trying to break them up will make none of you happy. Neither her, nor her current bf, nor you, because you can't get him out of her head 100% since she will always have her memories of and with him.

The only case where breaking up a couple is in my opinion acceptable is when their relationship is very loose anyway and when she herself is willing to leave him for you.