The most important thing with this exercise is that you pull the weight down to the front of your chest and not down the back of your neck. Doing pull downs to the back of your neck is an easy way to injure your shoulder joints.
To perform lat pull downs sit down in the seat and make sure your legs are securely in place. This allows you to use heavy weight while remaining stable. To effectively target the lat muscles lean back on a 45 degree angle to the floor as this will reduce the input your shoulders will have during the exercise. As you begin to pull down on the bar look up and pull down the bar as close down as you can to your upper chest. Control the weight back up to the starting position as you stretch your lats and repeat the process. Your grip on the bar should be a little wider than shoulder width apart.
Through this exercise move with your body's natural motion which means not being too strict on your form. Having said that, do not get too sloppy with your form to the point where you are jerking the weight.