How to let them out; start in late afternoon, so they will be motivated to get inside to their roosts in a short time. Don't start them outside in the morning, at least not at first.
Having them outside, you will loose birds, sometimes one, sometimes many at once. Everyone loves chicken! If you can't accept the risks to them, build a big predator proof run instead. In fact, build it anyway, because there will be times, sometimes for weeks, when they need to be inside under a roof anyway. Right now might be one of those times, depending on the AI situation where you live.
Chickens were jungle animals, and prefer to be under trees and shrubs to open ground anyway. Mine spend time under trees, and often run across an open pasture area to get to the cover of the barn. Mowed grass is not a favorite area either.
Mary