I have a maple seedling about a foot tall. It has not been planted in the ground yet, but it has changed color to its fall colours. It's is in a small pot. Since I may have missed the window to plant it in he ground, can you tell me what is the best way to save tree.. ? Can I plant it now, if so, what are musts.. And if I cannot, what is best way to keep it healthy?
Yes, go ahead and plant it. It's actually better for it to be in the ground over winter anyway, as it helps protect the roots. Planting trees in the fall is a common practice, your tree should do just fine. Just make sure it stays well hydrated, until the ground freezes, so you don't get winter burn. A light mulch (~3") of organic matter is beneficial too. Do not fertilize until the tree is well under way the following year. You can damage the roots, plus you don't want to push any new growth this fall.
Other than that, follow the general proper tree planting methods, and make sure it's got plenty of room.
Rule of thumb is you can plant trees in any month with an R in the name.
That means even December and January are good months to plant if your ground isn't frozen solid.
As J Musser said, fall is best. Not only does it protect the roots, the roots will continue to grow slowly over the winter and the tree will be MUCH better established before the brutality of summer begins.