What I noticed with my hand-raised chicks is that they just figured it out by themselves, sometime after they were fully feathered and out in the coop/yard full time. I'm a bit embarrassed to say that the first time I saw one doing it, I thought it was convulsing and dying, lol lol lol. So if yours are already in a place where they have access to dirt, I bet they'll figure it out on their own.
My hen-raised chicks started doing little dust baths much, much earlier, probably because they saw Mom doing it almost from day one.
Joni