If a girl really wants her ex boyfriend back, what should she do to get him back?

Tell him how you feel...
LOL, you didn't give away much info so that's the only thing I can think to say.
But I think the best way to go is tell him how you feel, Unless of course he dumped you and your harrsing the crap out of him
Then use the no contact rule and see what happens. You can read about no contact rule.. here in many threads.
Good luck.