my hens only sit on their eggs after they lay.

My hens do the same thing ( I guess they want some time with their egg before departing) but this is not broody behavior, if a hen is broody she will walk around with puffed up feathers ( often mistaken as being a sick chicken) clucking in a weird fashion. If you stick you hand in a broody hens box they will hiss puff up their feathers or peck at you.

 

She will sit on the eggs when she's "done" laying, once she's done with her clutch, she'll sit. Incubation period's about 20 - 21 days, she'll leave the nest to eat & drink, eggs will be fine.

 

Some breeds of chicken are more likely to go broody (hatch eggs) than others. You can't make them do so. You also need a rooster to fertilize the eggs or there won't be any chicks to hatch.