Sit down on the preacher bench and rest your elbows comfortably on the pads. With you palms down (reverse grip) grab the barbell at shoulder width apart and get ready for the initial curl. This is your starting position.
Curl the barbell up until your forearms are almost completely vertical. Then, lower the weight back down to the initial starting position but don't fully extend your arms straight. You should always keep tension in your arms/biceps for optimal muscular development and movement efficiency. Finally, repeat this entire movement for the desired or prescribed number of repetitions.
Perform this exercise in a smooth and fluid motion
Don't extended your arms completely on the downward portion of the movement
SPECIAL NOTES & LEVEL of DIFFICULTY VARIATIONS
To increase the level of difficulty, slow down the movement or increase the weight.
This exercise is not the ideal exercise for beginners since it does require fairly strong gripping power. If you're a beginner and still want do do this exercise, you may want to use a very light (short) barbell.
The good news about gripping strength is that it improves pretty fast. For some people it may take longer depending on how much previous weight-lifting experience you have.
In addition, forearm muscles get stronger and bigger fairly quick which means that you will need to change your forearm workout routine every 4 to 6 weeks to keep them challenged.