To perform a back squat using a Smith machine, stand with your feet shoulder-width apart and position the barbell across your upper back, just below your shoulders. Grip the bar with an overhand grip and unrack it. Keeping your chest up and core engaged, squat down by bending at the hips and knees, ensuring your knees track over your toes. Lower until your thighs are parallel to the ground, then push through your heels to return to the starting position. Maintain proper form and controlled movements throughout the exercise.