Before you fully start this exercise, take the tension in the band and 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
Pull the band till your elbows are as far back as possible and shoulder blades fully retracted