I'm not a fan of isolation exercises in general, but since it is your question, I would suggest putting yourself in a bent-over row position and perform reverse flys with dumbbells in a pronation grip Lower weight and higher rep is better since the rear delt is a small muscle and using too much weight will result in the activation of other muscles to compensate. 10-20lbs should be enough for this. This is a picture of the exercise: https://i.ytimg.com/vi/Tm_6m8iaVrM/maxresdefault.jpg Notice how his hands are sideways in pronation grip. Not sure why he's resting his head on a bench, that is un-necessary.