If you are lucky, your dog's nails will be trimmed like the above photo. But in the real world, things do not always go as planned. If you experience trouble, it will most likely be related to one of the following two issues:
You Have Cut Into the Quick
If your dog yelps in pain and the nail begins to bleed, it means you have cut into the nail quick. The blood vessel and nerve within the nail has been severed. Don't panic - this is not an emergency. Grab your styptic powder. If this is not available, use some corn starch or flour from your pantry (it is not as effective but it still helps). Use a cotton ball, tissue or paper towel to wipe away as much blood as possible. Get a pinch of the powder and quickly pack it onto the nail tip. Give your dog a break and reward him before moving on to the other nails. Though cutting into the quick does cause a sharp pain, the pain is not long-lasting and should not affect your dog's ability to walk. Use caution with the remaining nails, only cutting off small amounts at a time. Because the nail quicks grow with the nails, very long nails might also have very long quicks. Over time, you can encourage the quicks to shrink back by trimming a small amount of the nail tips every two weeks for a few months.
If your dog yelps in pain, but no blood is seen, it is possible that you are very close to the quick. Do not trim this nail back any further. However, some dogs dislike nail trims so much that they will yelp with each cut, even if the quick has not been cut. This is often because they have had their quicks cut before and are anticipation the pain.
Your Dog Just Won't Hold Still
It's hard to hit a moving target, and you don't want to accidentally cut your dog's nail quick. Some dogs will simply not stay still for nail trims, even with extra people helping. You can try working with your dog to gradually desensitize him to the nail trims. Reward him if he tolerates minimal paw handling, then gradually work your way up to nail trims.
If your dog is trying to bite you or is fighting so hard he could injure you or himself, then the nail trim is a job best left to the professionals. Fortunately, most vet offices and groomers charge a minimal fee for basic nail trims. You may be surprised how much better your dog behaves for a couple of strangers! Unfortunately, a small percentage of dogs will struggle too much even for the professionals. In the worst cases, sedation may be needed to trim nails. This is all the more reason to work with your dog or puppy to get him used to handling and nail trims BEFORE it gets that bad.