Grab the pull-up bar with the palms facing forward using the prescribed grip. Note on grips: For a wide grip, your hands need to be spaced out at a distance wider than your shoulder width. For a medium grip, your hands need to be spaced out at a distance equal to your shoulder width and for a close grip at a distance smaller than your shoulder width.
As you have both arms extended in front of you holding the bar at the chosen grip width, bring your torso back around 30 degrees or so while creating a curvature on your lower back and sticking your chest out. This is your starting position.
Pull your torso up while doing steps in air. At each step have a small pause. Simulate stair climbing action. Do that until the bar touches your upper chest by drawing the shoulders and the upper arms down and back. Exhale as you perform this portion of the movement.
The upper torso should remain stationary as it moves through space and only the arms should move. The forearms should do no other work other than hold the bar.
After a second on the contracted position, start to inhale and slowly lower your torso back to the starting position while still doing steps and pausing at each of them. When your arms are fully extended and the lats are fully stretched repetition is done.