You are talking about the lats.
You have to learn to isolate them first.
Do lat pulldowns on the machine. Arch yoru back and stick your chest forward. Squeezing your shoulder blades together, so you are pulling with your back, not with your arms, bring the bar to your chest and hold it for 1 second.
Slowly release and extend so much that you can feel the stretch in your lats when your arms are straight over your head.
Master this technique, and then you can move to other back exercise and construct a proper routine.