Choose an appropriate weight matching your fitness level. Take a stance with your barbell placed in the middle between your feet. Your stance is a little wider than your shoulders. Grab the bar with one hand, closer to the weight. Bend your knees slightly and bend over only as far that you can still preserve a straightened back while avoiding straining your lower or upper back and keeping your pelvis in a neutral position. The hand which holds the barbell is freely hanging, while the other one should not get in the way of your motion and help maintain correct back posture.
Flex your elbow, use scapular retraction to pull the barbell up from the starting position. Hold your elbow close to your torso as it leads the motion upwards.
Breathe out as you go up and breathe in as you go back to the starting position. Repeat as required. Alternate hands once you have completed the set.
Maintain the right posture, while avoiding straining your lower or upper back.
When you bend over, make sure the angle between femur and torso is around 90 degrees in both hips.
Your knees should not exceed the line of your toes.
Avoid uncontrolled sideways motion of your elbow.
Make sure you eliminate using your upper trapezius muscles.
It is best to perform the exercise in front of the mirror and watch yourself sideways to check for the right technique.
Choose an appropriate weight matching your fitness level; i.e., start with lighter weight to master the technique well.
The advantage of bent over one-arm long bar rows comparing to some other middle-back exercises is that the trainee can partially support his/her back by one hand. This makes it safer than, say, bent over barbell row but still less safe than seated cable rows or machine rows.
Make sure you practice one-sided scapular retraction when you pull the barbell up.
The knees should be slightly bent and the back as straight as possible.