Trim them about a week before the fair using fingernail trimmers or regular dog trimmers.
Look for the quick. It will be a pink area at the base of the nail. Trim as close to the quick as possible, but be careful not to quick them. They will bleed, but not to death. Just have some cornstarch or stiptic powder ready. The nails should be about 1/4 to 1/2 inch long depending on the size of the bird.
If you trim them between now and a week before the fair the rough edges should be nice and smooth naturally without needing to file them.
Don't forget about those beaks and spurs. Spurs just need the sharp tip trimmed off for the safety of you and the judge.
To trim long beaks just trim the top half of the beak so that it is the same length as the bottom half. The bottom half does not need trimmed.