I catch the chicken, hold it upside down and clip the claws with a pair of dog nail clippers. Just be careful not to cut too short and into the quick! There are also threads on this site that will tell you how to do it if you do a search to find them.
You need to put something in the pen for them to walk on that will help keep the nails short. I put those round cement pavers in near the door so they have to walk on it to get out. I don't have to trim them now. You also have to be careful to watch not to cut the quick. They bleed a good bit!
I agree with @welasharon however they cant always file them down enough, especially those whos toes were over grown when you got them or chickens like Silkies with extra toes, for them I trim their extra toes every few months min because they get over grown and can do a lot of damage if they fight