Every rear delt movement I see has the lifter start with his hands and arms in line with his body, then proceeding to move them outward (think of rear delt flyes). There’s nothing wrong with this, other than the fact that solely focusing on this kind of path severely limits how much of your muscle you’re going to hit.
To change things up, try setting up on a cable machine for constant tension:
This is a very isolated pattern that, when done correctly, absolutely torches the rear delts. It’s important to maintain the same elbow angle throughout the entire range of motion. This isn’t a triceps exercise; the fulcrum is the shoulder.
Pull the arm all the way across the body as though you’re trying to cover your mouth with your biceps. Then aim down toward your outside shoe with your fist. It’ll take a few reps to initially feel, but once you’re rolling, you’ll get an insane pump in the target region.