Select the appropriate weight matching your condition and type of training. Sit on the machine and place your feet shoulder-width apart. Grasp the handles with an underhand grip (radial side of your palms facing away from each other). Straighten up your torso.
Lift your upper arms all the way to your shoulders by flexing the elbows and contracting your biceps. Lock your elbows in one position on the pads. Then slowly return the weight back to the starting position in the same way. Breathe out as you go up and breathe in as you go back down to the starting position.
Avoid bending your wrists (dorsal flexion) at any time. Avoid uncontrolled muscle relaxation motion as you are coming down. Slowly extend your arms back to the starting position.
Start with lighter weight to master the technique well.
Biceps curls done either with pulley or with a machine are an isolated form of barbell biceps curls (more precisely, the movement is more limited and cannot deviate much from the trajectory given by the machine).
There are several different machines isolating the hand and enabling a proper elbow-flexing movement.
If you are using a pulley attachment it is advisable to use either one with V-shape or EZ-bar. This will put less strain on your wrist and lower the likelihood of wrist injury.
Isolated/limited exercises are always of questionable benefit. This is especially true in such a small muscle as biceps: isolation minimizes the involvement of important muscle groups surrounding the biceps and the main muscle is worked only from one angle.
The advantage is no strain on lower back (this is the case with most biceps exercises using barbell or dumbbells) and safety. Therefore machine biceps curls are ideal for elderly or people with back problems or those recovering from injuries.
When working on machine the weight should not be released completely at the end of the movement – you should keep the muscle tense during whole set.