Choose an appropriate dumbbell weight matching your fitness level. Your stance is a little wider than your shoulders. Grab the dumbbells with a pronated grip (palms facing down). Bend your knees slightly and bend your lower back as far that your torso remains upright without straining either your lower (lordosis) or upper back (kyphosis); your pelvis is in a neutral position. Keep your torso perpendicular to the femur. Your arms are hanging freely. Your palms are turned towards your body.
Lift the dumbbells to the chest (palms facing each other) by flexing your elbows and contracting your middle back (scapular) muscles.
Keep your elbows close to your torso as you lift up the barbell (not sideways). Go back to the starting position in the same manner (dumbbells touching your chest). Repeat as required. Breathe out as you go up; breathe in as you go down.
Make sure your stance is correct to avoid straining your lumbar lordosis (your lower back) and thoracic kyphosis (over-curvature of the 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 elbow motion to the sides.
Make sure you eliminate use of the trapezius muscles.
Perform the exercise in front of the mirror and watch yourself to make sure you are using the right technique.
Choose your resistance to match your abilities and fitness level. Start with lighter resistance to master the technique well.
Using dumbbells in bent over rows has several advantages: both sides of the body are working the same weights, the position of hands is less damaging for the wrists (palms are facing each other in the neutral position as opposed to the overhand position with palms facing down) and you can pull the dumbbells up in a slightly curved line.
Scapular retraction should be always done at the end of pulling motion. Keep the knees slightly bent. Your back must be straight and the head facing forwards during the whole set.