At What Joint In Tail Do We Need To Dock A Puppy?

This is not something that you should be doing even if you have medical training.  This is a surgical procedure that needs to be done by a veterinarian.  Tail docking needs to be done within the first 2-5 days after birth or it becomes a tail amputation.  In many states docking tails at home may be considered animal abuse.  The chance for complications rises dramatically when these types of procedures are not performed by veterinarians.  Appropriate local anesthesia and pain medications can be prescribed and use by your veterinarian.  
Please do not do anything at home that will cause pain for your pets.  Do NOT dock tails with rubber bands or any kind of constrictive material.  Do NOT use kitchen scissors or any other blunt, unsanitized device.
Although you will spend more money at a veterinary clinic the surgery will be done right, be legal, and will be the least painful for these pups.  Also if you do this incorrectly you will spend more money treating these pups than it would cost to have it done by a veterinarian correctly the first time--not to mention preventing these puppies from suffering.