You can commence loose leash walking as soon as you take your new puppy home. I recommend the use of a “Flat Collar” or “Gentle Leader” to prevent pulling and to help your pup to understand your expectations. And particularly if your new pet is a ‘Chocolate Labrador’. For healthy mental and physical development Labrador Retrievers should begin obedience training and particularly ‘leash walking’ immediately. 

To commence this discipline, fit a flat collar or gentle leader to your pup and allow them to walk and drag the leash. This will allow your pup to become familiar with the sensation of ‘neck/ collar pressure’.

The use of a ‘gentle leader’ allows for greater control when loose leash walking in high distraction environments. I suggest that you wear a ‘treat bag so that you can quickly refill your hand with high value treats. A quick delivery of the food ‘rewards’ gives you a high rate of reinforcement for the pup's loose leash walking. A high rate of food rewards will set your expectations and reinforce the routine in you pup’s mind. This will make it very easy to transition to a flat collar as your pup matures.

Your dog’s awareness of collar pressure is essential for teaching the controlled ‘onleash walk’. 

Position your pup to walk on your left side, at ‘heel’, on a loose leash with a flat collar or gentle leader. There should not be any leash tension or pressure on the dog's neck. 

To commence, take one step forward and if the pup follows, say "YES" and reward the pup with a treat. Your pup might become distracted and get out of position. Lure your puppy back to your left side and start again. The lure and reward will help you pup to memorise the routine. 
Continue taking one step at a time and mark the pup’s behaviour by saying YES and immediately reward with a treat.

You should teach your pup to relieve the pressure on its neck by giving them a short range of movement on the leash. 

If you hold the end of the leash your pup has much available movement before they feel tension.

Hold the leash in a comfortable position with either hand and in a position that keeps the pup close to you on your left side without tension on the leash. Holding the leash closer to the pup's collar allows you to respond quickly if the pup adds tension to the leash.

When you commence this training, your pup will be inclined to pull.

Don’t allow your pup any sniffing, scavenging, pulling, lunging, or leaping up when they are on leash.

If your pup is distracted and pulling, be patient.  Stop and be stationary. At this point, don’t redirect their attention, pull them back, give them a leash correction, or lure them into position. Your pup will eventually release the tension. When the pup releases the tension by stepping back, turning their head, readjusting their body, ‘mark and reward’ them. Mark the desirable  behaviour by saying YES and reward, then continue walking. Your pup will quickly learn  there is no benefit or reward in pulling on the leach.

When you teach your puppy loose leash walking, you are teaching the dog to respond to leash pressure. If your puppy offers a behaviour that adds tension to the leash, immediately stand still and wait. You can turn 180* and walk the way you came. You will generate leash pressure when you walk and this will teach your pup to release that pressure by following you.

Advance your pup when they are following in the heel position without lunging. 

If ‘you’ add leash pressure, reward your pup when ‘they’ relieve the pressure themselves. 

Dogs like to sniff and explore their environment, but we must teach them that this is unacceptable. Loose leash walking teaches your dog self-control.