1- First adjust the weights of the machine to suit you, and step on it on your knees.
2- Grab the pull-up bar with palms facing forward, wider than shoulder width apart, with arms extended.
3- Pull your body up until your chest reaches the bar level.
4- Lower your body slowly back to the starting point, and repeat.
Tips and Advices:
1- Concentrate on squeezing your back muscles while pulling your body up.
Perform the tough form of this exercise if you're strong enough Pull Ups.