First of all, I’d like to commend you for not just turning a blind-eye and for taking responsibility for this guinea-pig. Guinea Pigs’ nails need to be trimmed at least once a month to prevent them from becoming too sharp and interrupting normal behavior. In addition, long nails can get broken by accident and cause unnecessary bleeding. As the nail grows, the blood supply also grows, meaning that you are right to assume you can not just cut the whole nail off back to normal length. On pale-colored guinea-pigs it is relatively easy to spot where the "quick" stops – it is a more cloudy white area of the nail – but in darker cavies it is almost impossible.

Trim a little of the nail every few days so that the blood supply has a chance to recede, and continue doing so until the nail is of a normal length again. It is a good idea to have a little styptic powder on-hand in case you accidentally cut into the quick. Use a guillotine-style trimmer or human-style nail clipper as apposed to scissors to prevent fraying the end of the nail.