My broody had a nest in some tall grass. When I found it, it was about to rain really hard. I cut a door in one of those large plastic bins with a top and put some bedding in it. My husband picked her up and tucked her inside his raincoat while I quickly put all the eggs (18!) in the bin. I placed the bin in a small, quiet outbuilding with the opening facing a blank wall, about 4 inches away. We quietly settled her in on the eggs. In the morning I lifted the lid, and she was still sitting, in that trance-like state, so I moved the box further from the wall and placed food and water in front of it. About 5 days later, they started to hatch. Fifteen babies, and all still thriving free ranging with their mom a month later. I don't know if this would work with every hen, but it worked really well for me. I think the key is to do it at night. I once slipped a chick that hatched a day after all the others under the wing of a different hen at night, and she never noticed in the morning.
Mine didn't work out so well. After not having a predator problem for several months, something got her and 2 of her eggs. Later we trapped a raccoon. These are pictures of the 2 I hatched in an incubator. A boy and girl, the girl has the darker head. Buff Orpington for mother and Crested Polish for dad.