The pull up bar should be at a height that requires you to jump up to grab it; your feet should hang free.
1. Stand below the bar with your feet shoulder width apart.
2. Jump up and grip the bar with an overhand grip.
3. Bend your knees and cross your ankles for a balanced position.
4. Pull yourself up so your chin is level with the bar.
5. Lower yourself so your elbows are straight.
6. Repeat the movement without touching the floor.