Keeping your pups toenails trimmed is important to your puppy or dog's health. When a dog's toenails grow too long, it causes their foot to splay and makes it difficult, and even painful for the dog to walk. Even moderately overgrown toenails can affect the dog's ability to walk properly and can exacerbate existing joint problems.
When dogs have light or white nails, being able to see where the beginning of the nail bed is located is easy to do. However, for dogs with black nails you have to make an educated guess. As a general guideline keep just the tip trimmed. If you keep current with the nail trimming, you should never have to take more than the very tip off. If for some reason you trim too low and get into the nail bed, you will know because your dog will yelp, and bleeding will likely follow. For the yelp, gently pet and talk calmly to your dog, letting it know this was a mistake. For the bleeding, you want to stop this immediately, as dogs can actually have serious problems if the bleeding is allowed to continue.
To stop the blood flow, you can put about a teaspoon of baking soda or starch in a small dish or in the palm of your hand, formed into a mound. Holding the bleeding nail in the middle of the mound for several minutes allow the soda or starch to pack onto the nail, creating a cap that seals off the bleeding. If the bleeding is bad, you may need to do this several times. If you do not have either one of these substances - try baby powder. After trying several times, holding the nail in the substance for several minutes at a time, if the bleeding cannot be stopped, take your dog to a vet right away.
Reminder - It is also fine to take your puppy to a professional groomer or vet for their trimmings, but it will cost less and put less stress on your dog, if you do it yourself in the comfort of your home.