Have your puppy assume the a sitting position, hold a food treat close to his nose, and move it slowly down to the floor. Keep a firm hold on the treat, you can let him nibble at it a bit to keep his interest, but do not give him the full treat.
As his head approaches the floor, try and get him a low as possible. Once his head is low enough, it becomes easier for him just to lie down rather than sitting bent over. He will either stand or lie down, if he stands, you should get him back into the sit position before attempting to get him lying down again. Allow your puppy some time for this to happen, remain patient and try not to distract him by speaking or moving too much. If, instead of lying down, your puppy keeps standing, try to hold the treat a bit closer to his paws, reducing the amount of space he needs to move in order to reach it.
Be ready with the food treat, as soon as your puppy’s elbows touch the floor, let him have the treat and give him some praise. Give him positive attention while he remains in the down position by gently petting him and giving him praise. As you make progress, begin to incorporate the voice-cue ‘Down’ in a clear and decisive tone just as his elbows touch the ground. It may be beneficial to place a blanket or something soft under your puppy, as most pups don’t like lying on cold hard surfaces.
Keep practising the first 3 steps until he is lying down easily. When you feel confident that your puppy knows the command for lying down, you should continue practising, but without using a treat to lure him. If ‘Down’ is the voice-cue you are using, make sure that you do not confuse him by using other commands that contain the same voice-cue, for example, using ‘Get Down’ as a command to tell him to get off the sofa, or when he jumps up on people, will make teaching him more difficult.