Before you fully start this exercise, you should first bring yourself to an upright position. From this position it will be easier to lock your shoulder blades back, you should then bend your knees and descend by pushing your hips back (hinge at the hips). This focus on pushing your hips back will keep your spine neutral and put you in the correct starting position, in this position you should feel the tension in your hamstrings (back of your legs) and not the lower back
Keep your core strong and your spine neutral
Initiate the movement by retracting your shoulder blades (pinning your shoulder blades back), the muscles in the upper back should be the main focus and not just the pulling of the arms
Pull the band till your elbow is as far back as it can get and your shoulder blades are fully retracted