When cutting nails, it is essential to avoid cutting into the quick. Even though it can make your dog uncomfortable, cutting just the tip of the epidermis does not cause any pain. However, cutting into the quick will cause the nail to bleed and your dog will likely squeal in pain and run away. If that happens, apply a styptic pencil or powder to the end of the nail to stop the blood. Alternatively, apply gentle pressure for a few minutes with a clean tissue or towel until the bleeding stops.
Styptic powders are widely available at pet stores.
When the nails are black, it is impossible to see the quick. In these cases, cut small amounts off the tips of the nails every few days. Stop if there is any bleeding from the cut and start trimming again the following week. Alternatively, you can file off the tips with a rough nail file or even rough sand paper. Nails can become softer after a bath or swim so you may want to keep your clippers handy.