I don't like to mess with eggs. they can do it themselves. If you feel you must, I would wait 48 hrs.since you hear them peep, put a small hole in the large end of the egg very carefully. and then leave it for another 24 hrs. or longer. they cannot come out of the egg right after they start to peep because the air in the egg is not the same as on the outside, and they have to get used to it. If you help beyond that, go slow and if you see any blood STOP, and wait another 12 hrs. or you'll end up with a chick with a nasty belly button. My advise would be let it be and don't keep opening the bator they have hatched on their own for eons.
I set up my very last batch to hatch on their own while I was on a trip. I came home 2 days after they hatched, I had food and water in a large homemade hatcher and when I got home they hatched and figured out food and water on their own. it was sooo cool. yeah me!
You are likely to do more harm than good if you "assist" too early. Let it be and let Nature do her thing. They can take up to 48 hours (sometimes more) to hatch after they start peeping.