I integrate several batches of chicks into the flock every spring and summer. We have a large fenced run with a single gate. I do let the flock out to free-range, but only rarely due to a high predator load – coyotes, foxes, hawks, owls, eagles, raccoons, opossums, bobcats, and even the occasional cougar. I have four roosters to provide some security, and dogs in the fenced backyard howl their fool heads off when they see or hear anything amiss. Even the cattle help to keep predators at some distance. I still lose a few chickens every year; it’s inevitable.
I know they’re ready to explore outside the run when they’re nearly as big as the mature birds and have learned to follow the big birds' clues and warning calls. They also need to know the sight and sound of that coffee can with afternoon treats, and come running to it. I also watch for the time they’re freely mingling with the flock at feeders and treats, and don’t just stick with their own little group of hatchlings. Then I wait for a good stretch of nice weather when I’ll mostly be outside doing things for a few days, and can check on them frequently.
Only then will I open the precious gate to freedom. The big birds stampede through the gate and race each other to their favorite hangouts. The littles generally choose to stay inside the run at first, or explore just outside the gate for awhile. There are always a few hens and a rooster who choose to stay in the run or near the gate, anyway. (Especially if he’s got an eye on some of the pullets for his own harem.) I’ve found the youngsters don’t follow the flock to the far reaches until they feel comfortable doing so.
For the first few days at dusk, it's always a search-and-rescue mission until they learn how to find the gate into the run. I don't pick them up from wherever they've gotten lost (it's never very far at all); instead, I herd them (see below) back towards safety, sometimes with help, so that THEY follow the necessary route, at ground level, and find the gate themselves. It only takes about 2-3 days of this herding, and then they'll follow the big birds in or go into the run by themselves.
Herding chickens is a learned technique - and it's not as hard as you'd think, but it does take some patience and time. Basically, you act like a rooster would. He makes a certain sound that indicates it's time to head towards the coop. He circles around them and delivers light pecks to their behinds to get them moving. Then it's a slow, slow march punctuated by several pit stops to dine on last-minute snacks along the way. When the hens wander off the route, he circles back to them, stands patiently by and waits for a bit. Then he issues the call again, pecks or circles behind them, until they start moving again. Once they're in or very near, he repeats the process - because by then, several hens have wandered off somewhere else or back to where they started. It might take him ten round-trips to get everyone home.
I’m a bit more lazy (or more persistent), and will spend more effort keeping them all together and moving in the same direction, only going back for a second trip when one breaks away from the group. NEVER try to chase them; that never works – they’ll be panicked and race right past that open gate instead of going IN. You’ll wear yourself out and they’ll just be more confused and scared. They WANT to go in, they just need help finding the gate – and they’ll find it easier if you and they are CALM and take it slow.
My bedtime call is "Go to bed!" in a singsong voice the big birds have become familiar with. I carry my "rattly stick", which is really just a fiberlgass(?) stick with the end split so that it rattles when I shake it or tap it on the ground. Distinctive sound. I never tap (peck) the hens with it, but I'll tap it on the ground or against objects to get my point across. They always move away from me, and from my stick. So.... I just get behind or around them, and they move away from me and towards the run gate or coop door. We go very slowly, and I allow them their pit stops for last minute morsels, then it's tap-tap-tap "Go to bed!" behind them, and they start moving again. Works well for training littles to go into the coop at night, too.