In this exercise you should use the cable machine set the cable to the top of the machine.
Attach a straight bar to the cable and take hold of the bar with your palms facing downwards. Push the bar all the way down keeping your arms to your side as you come down and then come back up till your arms make 90 degrees and repeat this process in a slow and calm manner to get maximum benefit.
If you are just starting then start at the 3rd or 4th plate and try to move up by 2 plates every week if that is too much move up by 1 plate.
1st set do 3 or 4 plates 10-12 reps
2nd set do 5 or 6 plates 8 reps
3rd set maintain 5 or 6 plates 8 reps
4th set back to 3 or 4 plates and max out.
[image from workoutlabs.com]