Place your hen in a private place and put a wooden egg or two under her and see if she stays. If she does then place 1 to 6 eggs under her. Be sure you know they are fertile and start from that day. Leave her be only checking to see that she's sitting. She will get up once a day to eat and poop so don't be worried she's quit. She knows how long to be off and how to do what she needs to.
If you are unsure of the fertility, candle on day 14, at night and in the dark. Leave her be and she'll do the rest.
Mark your calender and let the stress begin. Your stress, she'll be as calm as a cucumber. Just don't do like some and go poking around under her. When things hatch just let it go. Some how she knows who's good and who's not and what to do with a bad egg. Listen for the peeping and watch for the fluffy head. Again don't go poking about under her.
They may not all hatch on the same day at the same time. Just keep an eye out that she's hasn't left the nest with the early chicks. If she does and you want just toss the unhatched eggs. If you want take the first chicks and hold them for a day and then put them back with her at night after your sure no more eggs will hatch.
Unlike an incubator I wouldn't let her sit more than an extra day.
Count the day you set the eggs as day 1. Even hens have their problems hatching so don't freak over a dead chick or shrink wrapped egg. Most likely there were problems with that chick. Weak chicks do not make good chickens.
Now keep us posted on how things go okay?
Rancher