The chin up is very similar to the pull up. Instead of using a overhand grip where your palms point outwards, you will be using an underhand grip where your palms point inwards. Because this puts the biceps in a stronger line of pull. Most people find chin ups to be easier than pull ups. Using a closer than shoulder width grip, grab the pull up bar with your palms facing inward. With your arms extended and holding the bar.

Bring your torso back around 30 degrees and stick your chest out. While exhaling. Pull your upper body up until your chin is just above the bar. The pulling is done primarily with your biceps and lats. pause for a moment and then return to your original position. Make sure to keep your elbows close to your body.

And just like in the pull up, your forearms should do no other work than holding the bar.