In a standing position grab the barbell with your palms facing up and 6 to 8 inches apart. The bar should be down and in front of you and your elbows should be tucked-in close to your body. This is your starting position.
Lift the barbell up until your forearms are almost completely vertical. Then, lower the weight back down to the starting position but don't fully extended your arms (you should always keep tension in your biceps by maintaining a slight bend in your arms at the bottom of the exercise movement). Repeat 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
Don't swing your body in an attempt to lift the barbell; miminal swinging is acceptable only if the weight load is really heavy
SPECIAL NOTES & LEVEL of DIFFICULTY VARIATIONS
To increase the level of difficulty, slow down the movement or load the barbell with more weight.
This is one of those exercises that puts on slabs of muscle onto your biceps really fast. If you're a beginner you can still perform this exercise, but with a lighter weight loaded onto the bar. You will find that your strength will increase very fast after you perform this exercise for just a few weeks.
The important thing to point out here is to keep your back straight with minimal leaning forward. Thus, to counter the weight of the loaded barbell pulling you forward and causing you to lean, you will simply lean backwards a bit. That's all there is to it!