Hold your leash with both hands, with the hand closest to the dog below the black handle part of the leash. How far down the leash you position your hand depends on how much freedom you want to give your dog. If you need to keep your dog close, you'd have this hand pretty far down the leash and close to your dog's head. If your dog can handle a little more freedom, your hand won't need to be so far down the leash.

This same hand should be positioned like you're riding a bicycle, palm down with the pinky finger on the outside, facing the dog. This dramatically improves your ability to apply the appropriate amount of pressure at the appropriate time, improving your ability to communicate with your dog.