Prepare the appropriate weight. Grab the barbell at shoulderwidth using an underhand grip. Kneel on the floor and put your forearms on the bench and let your wrists protrude from the bench (palms up).
From the starting position curl the barbell by bending your wrists upwards while contracting your forearm muscles. Do the required number of repetitions. Exhale while your wrists are moving up and inhale while they are going down.
Make sure your torso is upright and do not bend your chest (do not make a hunch).
Start with lighter weight on the barbell.