This article is from the General Cat Care FAQ, posted to rec.pets.cats newsgroup. Maintained by Cindy Tittle Moore with numerous contributions by others.
As an alternative to declawing and to help stem the destruction from scratching, many cat owners keep their cats' claws trimmed. This is easiest if you start from the beginning when your cat is a kitten, although most cats can be persuaded to accept this procedure.
Use nail clippers available at pet stores. Look for the guillotine type (don't use the human variety, this will crush and injure your cat's claw) and get blade replacements as the sharper the blade is the easier this procedure is.
There are also clippers that look like scissors with short, hooked blades. These may be easier for some people to handle.
Set your cat down securely in the crook of your "off" arm, with the cat either in your lap or on the floor between your knees, depending on the size of your cat and your own size. Pin the cat to your side with your arm and hold one of its paws with your hand (this is sometimes a little much for an "off" arm, you may wish to practice).
With its back away from you, it cannot scratch you, or easily get away. With your "good" hand, hold the clippers. If you squeeze your cat's paw with your off hand, the claws will come out. Examine them carefully (you may want to do this part before actually trying to trim them, to familiarize yourself with how the claws look).
If the claws are white (most cat's are), the difference between the nail and the quick is easy to see (use good lighting). The quick will be the pink tissue visible within the nail of the claw at the base. This is comparable to the difference between the nail attached to your skin and the part that grows beyond it. DO NOT CUT BELOW THE QUICK. It will be painful to your cat and bleed everywhere. When in doubt, trim less of the nail. It will just mean trimming more often.
Clip the portion above the quick for each nail and don't forget the dewclaws. On cats, dewclaws are found only on the front paws, about where humans would have their thumbs -- they do not touch the ground. Some cats are polydactyl, and have up to seven claws on any paw. Normally there are four claws per paw, with one dewclaw on each of the front paws. Rear claws don't need to be trimmed as often or at all; they do not grow as quickly and are not as sharp. You should be able to hold any of the four paws with your off hand; it will become easier with practice.
If you have too much trouble holding the cat still for this, enlist someone else to help. You can then pick up a paw and go for it. Be careful; this position often means you are in front of its claws and a potential target for shredding. Older cats generally object more than younger ones; this means you should start this procedure as soon as you get your cat if you intend to do this.
Trimming claws should be done weekly. Different claws grow at different rates; check them periodically (use the same position you use for clipping: it gives you extra practice and reduces the cat's anxiety at being in that position).
Claws grow constantly, like human nails. Unlike human nails, however, to stay sharp, claws must shed outer layers of nail. Cats will pull on their claws or scratch to remove these layers. This is perfectly normal and is comparable to humans cutting and filing their own nails. You may see slices of claws lying around, especially on scratching posts; this is also quite normal.