It needs to be outside so it can harden off and stop growing.

Has it been inside all along?

If it's been acclimated to the outdoors, wait until the leaves fall, plant it, then put a corrugated tree tube on the trunk, as tall as the tree is, if you like (2-3 ft tube?).

If the tree is not stiff enough to stay upright in the wind, use one of those slim, green garden stakes, driven in alongside the tube, and tie the tube to the stake with a string. That'll protect it from mice and rabbits and for the most part, deer.

should be fine.