I've clipped the nails of a friend's cat when she stopped wearing them down herself. They were almost complete circles by the time I got to them. She was elderly, and had never had her nails trimmed, but she didn't try to fight.

If you want to try it, human nail clippers work fine. Just clip the tips off. Cats' claws are clear enough that you can see and avoid the quick. My tiny dog's nails are all black. Also he is convinced that I'm going hurt him horribly. I trim a very little bit, frequently, and they're still too long.