First of all, if you really want to work on a weak point, give them priority in your workout routine. Schedule your shoulder routine to be right after a rest day, so that you come into the gym fresh and ready to hit the weights hard! You can also think about engaging in various intensity techniques to help you bring your rear delts up to par.
Some good exercises that really hit the rear delts include bent-over laterals, bent-over cable laterals, and face down incline laterals. Even barbell rows recruit a lot of muscle fiber from the rear delts, even though they are primarily a back exercise.
With these exercises, use the strictest technique you can to avoid heavily recruiting muscle fibers from other muscle groups you're not specifically working (i.e., no cheating!). Even if you have to use lighter weight, try to always let those delts (namely the rear delts) do most of the work.
As far as sets and reps go, if you're going mainly for muscle growth, I'd recommend 8 total sets of exercises geared towards rear delts, and 4-6 more sets of a heavy compound shoulder movement, such as military press, clean and press, heavy upright rows, dumbbell presses, ect.. This way, you're giving priority to your rear delts, but you're not neglecting to train the shoulder as a whole (even though any shoulder movement is going to involve every head of the deltoid to a degree). As far as reps go, I'd recommend anywhere from 8-12, maybe a little higher or a little lower depending on the individual and their specific goals. For example, for the first set, you could shoot for 12-15 reps, the second and third could be 10-12, and the last one could be 8-10, progressively increasing the weight as need, while maintaining strict form. And you can come up with many different variations. The best way is to experiment a little and see what suits you best!