Stand up and grab a hold of the bar no more than shoulder width apart. Your arms should be down and in front of you. You should be far enough from the machine to allow you unobstructed execution of the exercise (you don't want to be touching the vertical cable line when you are lowering/lifting the weight. This is your starting position.
Lift the weight up until your forearms are almost completely vertical and just a few inches away from your chin or neck. Then, lower the weight back down in front of you but don't lock out (i.e.: don't fully extend) your arms at the bottom. You should always keep tension in your biceps. Finally, 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 weight with less effort; this is cheating and doesn't accomplish anything
SPECIAL NOTES & LEVEL of DIFFICULTY VARIATIONS
To increase the level of difficulty, slow down the movement or increase the weight.
This particular exercise is the cable Preacher version of the bicep curl. It does require a few minutes to set up but it's well worth it.
The cable motion provides for a smooth motion while the preacher fixes your arms into a set position that allows maximum targeting of the biceps.
The only downside to this exercise is that it requires you to haul the Preacher bench over to the cable apparatus. And for those of us who have done this before on many occasions, we know that's no easy feat because Preacher benches are usually heavy. In some gyms they are bolted to the floor, so you can't move them around at all.