You just need to plant them where they have room to grow!
You could transfer them to pots for their first year or two, or plant them in the ground where you want them permanently.
It's likely that there will be quite a bit of variation among the seedlings, in terms of leaf color, fall color, branching habit and so on. If you have a spot in mind, I would plant 3 or 4 pretty close together, like a foot or two apart. You can watch them develop for a couple years, then pick the one you want to keep and cut down the rest.

If you have rabbits in the yard, I would put some kind of protection around them, like a chicken wire cage, especially in the winter.

Alex