How to cut your dog’s toenails yourself
Dogs with long toenails are similar to women wearing high heeled shoes whenever walking or standing and eventually contribute to knee joint injury.  Long toenails essentially do the same to dogs by changing the natural alignment of leg bones which connect all the way up to the spine. The adjustment of the way they walk due to excessive long toenails will lead to shoulder pain, hip joint pain or even spiral injury.  Your dog cannot cut their own nails, so it is our responsibility to keep maintaining before any injury occur.  Remember, Remember that trimming claws is not merely a cosmetic issue, but it is also a health issue.
It`s easier to cut on a lighter colored nail. The bottom pink part is the quick (the blood vessel). Don`t cut the quick, It will bleed! The nail starts to curve where the quick ends. Cut the nail, at a 45 degree angle, a bit away from the tip of the quick or where the nail starts to curve. 
Picture
There are different size of clippers for small and larger dogs. For toy breeds, sometimes a human nail cutter can do as long as the dog feels comfortable.
 PictureStyptic Powder. Use a Q-tip and apply enough to stop the bleeding when cut too short.
Some dogs hate nail trimming, almost none like it. Some dogs need tranquillizers to make it through the process without biting. No matter what your dog’s personal take is on nail clipping, it is something you should do for your dogs regularly to keep from harming their skeletal structure.
Before your first time cutting the toenails, practice with your dog, repeatingly, by lifting his legs, holding it and touching the nails. Then let go. Let the dogs get used to being handled. .   
Restrain your dog in a secure position. If your dog is fighting hard, wrap him in a towel and get a second person to help. Have something like styptic powder to stop bleeding, just in case.
Cut one leg at a time would be less stressful sometime for both owners and dogs. Sometimes it works even better to cut one toenail at a time. 
Frequent nail trims let both you and your dog get used to doing them and eventually become a routine.
Remember, the quick grows longer as the nails gets longer. Frequent trims will regress the quick.
Cutting it right on darker nails need practice.  Try to apply the same technique of cutting light colored nails as mentioned above. The nail in the picture needs to be cut at least half of the nail`s length. (Left: Before, Right: After) 


At the end of the day, getting a professional to do the job is always a affordable option.