Start by gripping the bar with hands in an overhand grip. How wide your grip is will depend on your body's dimensions, but you'll want to aim for having your hands just outside shoulder width. If you use too narrow a grip you'll end up shifting the focus from your lats to your forearms.
The target point of the lat pulldown is extremely important for proper form. Your goal is to target your upper chest muscles around the height of your collar bones. It's important not to cave your chest in forward and own while pulling the bar down. Instead, you want to visualize bringing your upper chest to the bar as you pull down.
That's pretty much it.
Have fun!