Hello and thank you for your question. If you aren't sure about how to trim your dog's nails, it would be best to have someone like a vet or a groomer give you a demonstration. Dog nails have a blood vessel inside, so if you cut too far back into the nail, you can cause it to bleed a lot and it is painful, so they will be very resistant to future nail trims. Also, if they have not been trimmed for a while, the quick (blood vessel) is usually pretty long and you can only really trim a tiny bit off the ends. If you do this often, though, like weekly, you can often slowly cause the quick to get smaller and over time, you can get the nails back to the regular length. They should be cut monthly as a routine.
Someone who is used to doing nail trims would be able to look at your pet's nails to tell you if you can safely cut off a large portion of the nail now or not... If your pet's nails are light or white in color, you can see the pink of the quick, but if they are black, then you can not tell by looking where the quick is. That is why it's helpful to get someone who's comfortable doing it help you through it. Most vets or vet techs will gladly give you a demonstration of how to properly cut your pet's nails.