Concentrate on the feel of your lats as you move the weight. Try different planes of motion (to waist, to chest, to somewhere in between) until you fell it in your lats. Try stretching out farther at the bottom, or holding for a 1 second count at the top, or ... Just keep experimenting and doing them until you get the feel you are looking for. Also, be extremely strict, don't use a torso twist or jerking motion, or you will place more strain on your lower back or abs or something other than your lats.
BTW, your lats are a much larger and stronger muscle than the rear delt so the tendancy should be to use the lats if you are using a heavy enough weight to challange your lats.