They may start laying eggs outside the nest but usually they know what to do. I always liked to put golf balls or ceramics eggs in the nests just to give them the right idea. Not sure they need it, or even if it helps but that is what a lot of folks do.
X2 with chook. I use golf balls. Don't think it's too too young to put them in there.
If you get a couple outside the box, don't get distraught. Sometimes the first eggs are a surprise not only to you but also to the birds. They'll eventually get the idea.
I guess the question is, when you say not using, does that mean not laying or does it mean laying outside of? Depending on the breedor bird, not laying yet is totally normal. If they're already laying and doing it outside the box, decoy eggs may help.
Last time I modified a coop I made 4 or 5 nesting boxes that were at least 12 inches higher than the main floor. All worked very well, just once in a while had a floor-laid egg. In the coop I have now, with my first run of pullets not yet laying, the nesting boxes are at floor level, separated only by a 2-inch board at the edge. I'm wondering... will they realize that these boxes are nesting boxes? It would be nice, as the roof above the boxes is hinged to allow easy egg gathering.
They won't be laying yet so naturally they won't be using the nest boxes yet. Like the others have said golf balls and/or ceramic dummy eggs work the best to teach them that this is the place they are supposed to be laying.