Before you start clipping, determine how much to trim. The basic rule of thumb is that the nail, which curls downward, should be even with the paw pad. Whatever hangs over needs to be clipped. The earlier you start clipping your kitty's claws, the more accepting of the ritual she will be. Frequent trims when your cat is young will help diminish fear. Use the proper instruments. There are a variety of nail trimmers available at pet stores or your veterinarian's office.
For more information, please read the story Trimming Your Cat's Toenails.