Expert players like those featured on YouTube suggest these drills:
Disable this. It can interfere with the quick release of Renner's shots. Beatrix Auto Aim Sniper
Hold down the Renner basic attack button to bring up the long-range sight. Expert players like those featured on YouTube suggest
Aim slightly ahead of where the enemy is walking. Since the bullet has travel time, aiming directly at them will often result in a miss if they are moving perpendicular to you. Beatrix Auto Aim Sniper