Do this movement kneeling or seated depending on what is available at your gym. Before pulling down get the elbows straight and glutes tight. If you are doing the kneeling variation, the shoulders, hips and knees will be in a straight line before pulling the bar down.
Bend the elbows and pull the shoulders blades down and back to lower the weight. The posture should not change during this motion.
Pull the bar down to the collar bone while keeping the elbows under the bar. A common fault is to let the shoulders raise up towards the ears and round forward at the bottom of this movement. Prevent this by thinking about putting the shoulder blades in your back pocket and pushing the back of the armpit forward. Once the collar bone is reached, slowly lower the bar back to the starting position.