A chin is very similar to a pullup. The only difference is that your hands are reversed. Your palms will be facing you with an underhand grip.
Grab the bar shoulder width apart and hang from the bar. Without a kip or swing pull up till your chin passes the bar and then lower yourself controlled until you are back at the starting hanging position.
To mix it up and hit different parts of your muscles, try close grip and wide grip as well. Same exact form but just moving your hands closer or father apart.