Begin by ensuring you have an elevated box or bench to stand on. Jump up to grab hold of the bar with an neutral grip. Make sure your hands are shoulder width apart and elbows are close to your body. You should begin with your chest is nearly touching the bar. Then proceed to lower yourself as slowly as possible until your full bodyweight is hanging from the pull up bar and your arms are fully extended. All the while inhaling as you do so. When you've reached full extension, let go of the bar and step onto the elevated box or bench you had in place. Repeat.