Or are you already able to perform pull-ups and are looking to fine-tune?
Let me introduce band resisted wall climbs.
This exercise develops the controlled mobility of the shoulder blades.
Being able to own the movement of your shoulder blades will absolutely improve your ability to perform pull-ups, and will help your body feel great while doing so.
Let me be clear. This exercise is not for your arms. It is ALL about the muscles that control the movement of your shoulder blades.
So your arms should be performing very little work.
Place a resistance band around your wrists. Choose a tension that allows you to use proper form at all times.
Adopt a split stance and put one foot against a wall. Your torso should be 3-6 inches away from the wall.
Bend your elbows to approximately 90 degrees, set your forearms so they are in a vertical position, and place your forearms against the wall so they are chest height.
Now press your arms out against the band. When you do this, you should feel the muscles around your shoulder blades kick in.
Even though your arms are pressing against the band, keep your arms and forearms as relaxed as you can, as this will allow the muscles around your shoulder blade area to do their thing.
Now engage the muscles around the shoulder blade area, and slowly walk your arms up the wall. Once you reach your end range, walk your arms back down the wall and return to the starting position.
For the duration of the exercise, make sure that your arms remain in a vertical position, and do not allow the band to lose tension.
For the duration of the exercise, your head, torso, and hips should remain in a stacked position. Do not allow your lower back to hyperextend or your ribcage to flare.
As for your breathing, do what feels the best for you. Be sure to take deep breaths in through your nose (360 degrees of air around your spine), and exhale through your mouth.
Make this exercise easier by using no resistance band, sliding the band down your arms so it is closer to your elbows (versus your hands), or using a band with less resistance.
Make this exercise more challenging by using a band with more resistance, using two bands, or increasing the range of the movement.