can't be too hard. just look for any screws behind/under the door handle, and around the edges of the door panel. any round or square/rectangular plastic inserts in the door will have screws behind them. once you've got all of the screws out, it should either pop straight off, or lift up to come off. i'll look around and see if i can find anything.
i have no mustang. so don't listen to anything i say.