Select the appropriate weight matching your condition and type of training. Grab the EZ-Bar with an underhand shoulder-width grip. Position your upper arms on the front bar rest from shoulders to elbows. Sit down on the machine with your feet shoulder-width apart. With the upper arms positioned against the preacher bench pad and the chest against it, hold the EZ-Bar at shoulder height. Keep your wrists firm.
Use bicep contraction and elbow flexion to curl the weight up to your shoulders. Lock your elbows in one position on the pad. Go back to your starting position in the same way. Breathe out as you go up and breathe in as you go down.
Avoid bending your wrists (dorsal flexion) at any time. As you are sitting make sure your torso is upright to avoid straining your lower back (lordosis). Avoid uncontrolled motion as you go down. Breathe in as you extend your arms and perform the motion slowly.
Start with lighter weight to master the technique well.
Preacher curls are one of the best examples of isolated/limited exercise. As such they are not the best mass-gainers. The advantage of preacher curls is that the position of your body is forcing you to do the exercise precisely, without cheating and without moving your elbows.
The added value of preacher curls for the beginner is questionable. Advanced bodybuilders can do them for a better definition of their biceps muscles.
EZ-bar is preferable over a straight barbell – it puts less strain on the wrists that are in unnatural and possibly dangerous position.
Alternatively, preacher curls can be also performed with two dumbbells.