Do not do anything until 24 hours after the chick pips (breaks a hole through the egg shell). Before that you can poke a hole to make sure the membrane and egg both have a hole so the chick can breathe. Trying to help sooner than 24 hours results in too many failures with dead and misshapen birds.
After 24 hours you can help if you want. Some will die but you can save some.
The first thing to remember when helping is to watch for blood. If bleeding starts (not just the small blood in the membrane), STOP right then and there. Put the egg back and wait a few hours before again trying to help.
Once you start helping a chick, it often won't be able to finish by itself. That is because when exposed to air the membrane dries out and gets tough. The air also signals the membrane's blood vessels to shut down and that is good.
The easy ones to help are the eggs where the chick has cracked all around the egg. Sometimes at this point the membrane dries out and gets too tough for the final exit. Just gently split these eggs open.
The hard cases are the ones with just a pip. Start at the blunt end of the egg and fold the membrane down over the chick. In the ideal case you end up with the membrane rolled back to the chick's butt. At this point you can put the chick back in the incubator/hatcher if the egg sack is still not absorbed or if the chick is very weak.
Be careful when putting a chick back. At this stage some can be very sticky and will glue itself to the wire bottom in the incubator. If the chick feels sticky, put it on a piece of wax paper.