Same idea as everyone else who does it successfully - "see and be seen, with a partition between". I hatch every 3 weeks (chickens) or 4 weeks (ducks) [plus a day or two to clean everything out and collect enough eggs to restart]. So I'm integrating new hatchlings into my adult flock every single month.
When born, they move to the brooder box - which is just that, a big plastic tote. As soon as they are too big for that, they either move to the back of my shed and a PVC temporary run, or to the grow out pen and run inside/outside my barn. Its there that they begin the see/be seen process in earnest, and they begin between 3-4 weeks. By six weeks, I want to have them free ranging with my adult flock - and with the exception of a single duckling last year, I've been successful at that.
Overnight, I have two adult runs - one of which has some netting around the bottom to keep the "littles" in. That's where the most recently integrated, and the integration before that, plus the youngest ducks sleeps (12-25 birds) - as well as any injured fowl. Everyone else is in the big pen with the goats (30+ birds). But they all free range together all day - and frankly, the biggest adult birds can (and sometimes do) hop onto the gate to jump out of the big run, then do it again to hop into the little run.
If I had more adult hens to occupy my drake, I wouldn't need two runs at all - but right now he tries to jump adolescents, he's so randy. Its a problem.