The bar should be low enough that your feet remain on the floor while you grasp the bar with your knees slightly bent. If it isn't, stand on a platform.
Keep your thumbs on top of the bar. A lot of people like to wrap their thumbs around the bar, and they end up losing a significant amount of leverage and pulling strength.
Stand below the pull up bar. Grasp the pull up bar with an overhead grip, with your hands over the bar and approximately shoulder width apart. The bar should be low enough that your feet remain on the floor. If it isn't, stand on a platform.
Extend your arms all the way down from the bar while bending your knees into a slight squat.
Use your legs to propel yourself off the ground while simultaneously pulling with your back and arms.
Raise your body up until your shoulders are above the bar.
Lower yourself back to the ground by extending your arms and placing your feet back on the ground.
Do not swing your lower body to gain momentum and make the exercise easier. You're much better off doing a few perfect form pull-ups than a lot of cheating pull-ups.
Do not keep your thumbs below the bar, you will lose a significant amount of leverage and pulling strength.
