Select the appropriate weight matching your type of training and your fitness level. Take a shoulder-width stance. Grab the bar with an underhand shoulder-width grip (radial sides facing each other). Stand up with your torso upright and a dumbbell on each end of the bar being held an arm’s length away.
Curl the bar up until the forearm touches your biceps (bicep contraction). Keep your elbows close to your torso and stationary all through the movement. Go back to the starting position in the same way. Breathe out as you go up and breathe in as you go back to the starting position.
Make sure to maintain correct stance and bar grip. Avoid excessive elbow motion in the sagittal (front-rear) axis and undesirable dorsal wrist flexion. Maintain correct stance without bending your back backwards (lumbar lordosis).
Watch yourself in the mirror to maintain a correct stance in all stages of your motion.
To acquire better control and the correct stance, you can start this exercise by leaning against the wall to avoid excessive elbow movement and rocking.
This is the most common variation of barbell biceps curl.
Barbell curl is a basic biceps exercise. Its main disadvantage is the unnatural position of the wrist that can cause problems especially with higher weights.
This is why many athletes prefer EZ-bar over straight bar.
The closer grip will strengthen and build more the outer part of biceps muscle (the long head of biceps brachii and brachialis muscle) while the wide grip is better for the inner (short) head of biceps brachii.
Do not cheat in first repetitions. Try to do the movement slowly; you can apply some cheating in the last two reps.