The end of your story, that is, the third act, is easy if you’ve done your job in the first and second act. There you’ve set up an ending. Just do something different that fits the clues and your golden!
The greatest problem with an ending is when it’s predictable. But the opposite problem is also detrimental… the ending that comes out of nowhere.
Your third-act job is to give the reader the ending they expect in a different form than they expected. Sometimes that’s just giving the end a twist. In a romance, for example, you might set up that the heroine is going to jilt the guy and marry the fellow next door. Instead, she discovers something she didn’t understand about the first guy that makes her want to keep him. In a thriller, the bad guy turns out to be someone other than who we thought it was.
Alternatively, as the writer, you hold back a critical piece of information from the reader that skews the expected ending to one more apt.
As important as the conclusion is, it’s always the setup that’s important. When does the end begin? When your character is fully outfitted for the fight. Whatever character flaw is addressed, whatever relationship is mended, whatever weakness – internal or resource – is fixed or acquired. In act one and two, the hero is no hero, by the third act, the hero IS the hero (use the term lightly; your main character may be a creep, but he’s the creep on the right side now).
All the conflict of the second act adds a plate to the hero’s armor, refines character, increases the stakes, offers red herrings and reversals, and narrows the story to its conclusion.
It’s tempting to achieve the ending too fast. Conflict and surprise need to be part of the process, something that puts the reader in doubt of the expected ending, but the last thing that should happen is that the hero’s plan actually works. Plans fall apart; character and improvisation makes it work. Short third acts are common in today’s fiction. Don’t fall prey to the idea that the first and third act should be roughly the same size. Sometimes dropping of the cliff (a short third act) is ideal. It’s only rushed if it feels rushed (my first novel’s ending is a bit rushed. We write, we learn.)
Classically, there are two endings: Comedy, where things end well for the hero, and Tragedy, where things end badly for the hero. (There are also tragedies that end up working out for the hero… he loses the girl, but she’s not what she seemed…) There is a lot of wiggle-room in those classifications. True tragedies are rare today, but they can still work.
After the pay off, we then need to see the cool-down phase after the ending. The sigh of relief, the French story term I can’t remember how to spell that sounds like day-num-wa (I’m so pitiful sometimes). It can be right after, or days or even months/years after. It’s a time of implied reflection, a taste of the new normal.
A final comment in reflection of the Amazon pool of reality and POD: Your story doesn’t have to be 90,000 words. It can be just about any size now (at least 25 pages for POD printing). The terms novella and novel have little meaning anymore. In printing, 200 pages was necessary for a good price-point. Not so anymore. All that to say, end your story when it’s ready to be ended. Is it “too short” but feels satisfying? Then end it. If it doesn’t feel satisfying keep working on it.
Who doesn’t love endings? It’s all downhill and craft at that point. You’re almost done!