JillA, we had a bantam years ago with this prob, one of the farm chaps popped up and filed it with a small file. He told me to never cut but to rasp. I understsnd that there may now be newer methods, but this did really do the job
I wouldnt do it yourself, there's a reason its done in day olds in commercial flocks and a reason that it will be phased out over the next few years. I had a hen with a slightly twisted beak which then overgrew on the top-she still managed to eat fine and live to a ripe old age but I used to find chucking some corn down on the tarmac drive would file the end down a bit. If its interfering with eating and preening then take her to a decent farm/bird vet.
Well I did it today with some nail clippers. When I had someone to hold her and had chance to have a good look she had what was like a little stick of white cartilage sticking out from her lower beak where it didn't meet the upper, (separate from the beak itself) which I removed bit by bit with the clippers, and another very small piece to the side of the upper beak. It was quite easy to see which was beak and which was cartilage, and hopefully she will be able to wear them down herself for a while.