Choose an appropriate dumbbell weight matching your type of training.
Take a shoulder-width stance with your torso upright, your feet straight facing forwards.
Grab your dumbbells with your wrists in natural position, i.e. palms are facing each other (inwards) throughout the whole exercise. Keep your torso upright.
Curl the weights forward towards your shoulders without changing the rotation of your wrists. Keep your elbows close to the torso throughout the entire exercise. Go back to your starting position in the same way. Alternate your arms. Breathe regularly throughout the whole exercise while inhaling in the starting position and exhaling with every bicep contraction.
Choose the dumbbell weight to match your abilities, fitness level and training type. Start with lighter weight to master the technique well.
Make sure your shoulders, knees and feet are at one level.
Focus on your abdominal breathing.
Maintain an upright position without bending backwards at the waist. Only the forearms should move, not your elbows.
Choose the number of repetitions to match your training type.
Avoid bending backwards at your waist by using lighter weights or leaning against the wall.
Watch yourself in the mirror to maintain a correct stance in all stages of your motion.
Make sure you eliminate using the trapezius muscles.
Frequency of repetitions depends on correct breathing and technique.
This is a variation of hammer curls: while it is sometimes hard to coordinate the movement of both arms, alternate movement enables more concentration. Some athletes do the alternating curls in the last set (when the muscles are already exhausted) or only at the end of each set (for the same reason).
Hammer curls are curls in which the palms are facing each other. This is the most natural position of our hands thus causing least wrist problems.
Another advantage of hammer curls is that they build brachioradialis muscle that gives the appearance of volume to the forearm.
Hammer curls are usually done with two dumbbells; there is also a special barbell designed for hammer curls (unfortunately not available in every gym).