I would allow them access to the nesting box from the start. Just clean up any poop! That way they see the safe place to lay eggs and learn from early on.
When one of my hens was broody but had no fertile eggs, I gave her two newly hatched chicks from the stockfeed shop. For the first few weeks I kept them in a tent I nicknamed the "maternity ward", but found it was not necessary as their mother fiercely protected them from the rest of the flock. So I started letting them out - their mother took them to roost inside one of the nesting boxes while the rest of the flock roosted up on the perches. I just cleaned up any poop the next morning.
Pretty soon though the chicks were roosting with their mother up on the perch! Then after a few more weeks they roost with each other but away from their mother.
Meanwhile I had some 3-month old pullets who would see the hens laying their eggs in the nesting boxes. By the time they reached six months old they learned to lay in the nesting boxes too. I only ever found three 'rogue' eggs in odd places away from the nests.