To get the most out of the chin up, do it slowly and use a full range of motion.
Control the movement. Dont let yourself drop too fast or swing your body to help yourself to get all the way up.
This exercise can be made easier by using a chair or a spotter for assistance.
Instructions
Chin ups are a great exercise to hit the back and biceps. The underhand grip means the biceps are involved much more than in during a pull up. Grip a chin up bar using an underhand grip with your hands about 6-10 inches apart.
Lower your body down so that you're hanging from the bar. This is the starting position.
Slowly pull yourself up until your chin is above the bar, and then lower right back down to the starting position.