The ending of your story is where the final dramatic scene takes place, in a spectacular story climax. And then it's the place where your reader - who has lived through this adventure along with your characters in a state of (we hope) tension - can relax at it all being resolved.
The structure should be something like this:
After the scenes consisting of the rising action, there should be one final dramatic scene, the climax.This is the biggest challenge your protagonist will face (but as I write in The Middle of your Story, he will have grown and developed (i.e. the character arc) through meeting the challenges leading up to this moment, and so there’s a good chance he’ll be strong enough to win. It’s not guaranteed, of course, because that would be too easy, and not interesting for the reader. But at least it’s an equal match.
This biggest challenge will usually be a final confrontation between the protagonist and the antagonist. The classic example of this is the meeting of James Bond and the villain, and James’ escaping whatever death trap has been devised. If the 'antagonist' is an inanimate object - a volcano for example, then the climax will be the most dangerous encounter with it.
After the climax, there will be a scene - or scenes - known as falling action. This is where the dramatic question is finally answered - hopefully to the satisfaction of both the characters and the reader; the subplots are all resolved; and all the threads of the story come together. This stage is sometimes known as the dénoument, which is the French for 'unknotting'. It's as if the plot was a tangled mess, and it's now neatly straightened out.
This is the stage of the story where the reader can enjoy (as, indeed, can the characters) the fruits of their victory - the lovers spend some time together, the cowboy rides off into the sunset, the thieves in a heist story contemplate their new wealth.
This denouement section really should be included, or at the very least, hinted at. It would be too much for the reader to come all this way and invest so much, without being able to vicariously enjoy the pleasure of success.
However, this section mustn’t be too long. If it’s too long, it’ll be boring. The reader thrives on conflict, and there’s no conflict in the ending of the story. So, let the characters and reader celebrate briefly, and then end the story.
Fairy tales do this very well, in their simple statement that they all lived happily ever after. We don’t need any details, we don’t want to hear the minutiae of years of domestic bliss. But we do like to know it happened.
The ending of the story is the easiest part in many ways. I’ve explored the respective challenges of the beginning of your story and the middle of your story, and the ending doesn’t pose any such difficulties.
The only bit you need to get right are the climax, and the resolution of the varying threads in the story. The rest almost writes itself, and it’s a pleasure to write. Not only do the characters and (in time) the readers get the pleasure of having successfully taken this journey - but so do you, the writer.
Having said all that, depending on the genre, you mightn’t tie up every loose end. You certainly have to resolve enough plot lines to satisfy the reader, but it might be just too twee to have them all sorted out. After all, real life isn’t like that - we never get total closure and perfect symmetry in real life.
So, in fairy-tales or fables, it can all end perfectly, but in contemporary fiction leave a few minor threads open. You can hint at their possible resolution without categorically stating it.
Try to have the characters say something which refers to the future, to show how their lives will continue after the ending of the story, in much the same way as paths in paintings always head out of the frame.
One last thing to say: if you find that the climax doesn’t come together, the threads refuse to tidy themselves neatly, or the plot can’t be resolved - then it’s very possible that you’ll have to go back and seriously re-write earlier parts of the novel in order to put the situations in place which allow the correct climax.
This most likely won’t be a problem for those who outline but the wingers can find themselves in this situation. (Although, I have to say that it’s equally likely that the wingers will find a perfect solution presents itself, better than they could have thought of themselves!) In either case, you might well find that using good writers' software will help avoid this problem entirely.