Set-up: Find a pull-up bar, or another immovable graspable object. A barbell locked in place or sturdy desk may also suffice - simply make sure it is immovable and unbreakable, to bear your weight without incident. Face the bar or hold and grasp it with both hands, such that a line forms between the shoulders, the elbows and the apparatus.
Lift: Keeping your back flat, lift your feet off the ground. Raise your body until there is a straight line between your ankles, knees, hips and shoulders, and you are parallel to the floor. Holding this position, use your back and arms to pull yourself up until your chest touches the apparatus. Lower under control and repeat.