The length of your dog's nails directly impacts their posture, movement efficiency and comfort when walking and running. Long nails cause pain when a dog puts weight on all four paws, and so dogs tend to shift their weight back to relieve pressure on the nails. This change in posture can lead to discomfort or even pain in the spine, shoulders, muscles and joints. With very long nails, the change in posture can cause negative orthopedic changes as a dog ages.
The hyponychium (also called the "quick") is the epithelium located beneath the nail plate at the junction between the nail shell edge and thicker structure underneath the nail. The images below feature my dog's unclipped nail, to show you exactly where the quick begins and what section of the nail is safe to cut.
The line where the structure of the underside of the nail changes is your safety line - do not cut past into the lighter colored fleshy underside of your dog's nail.
You want to cut the black hollow shell portion of the nail, before the quick / fleshy underside of the nail. For an efficient clip, you will want to make TWO cuts to the nail. One vertical (red), and the other on a vertical slant (blue). This method of trimming will help the quick recede and allow you to cut your dog's nails shorter the next time.
Do not cut too much off the top on your second (or blue) cut, you just want enough to help encourage the quick to recede.
Nails should be trimmed once a week, or once every two weeks depending on how fast your dog's nails grow.
Nail trimming is especially important for dogs with joint issues, any kind of bone/muscle pain, and senior dogs.