Some people give birth to wonderful endings in one burst. In fact, many writers only begin work on a story when they imagine a full and powerful ending. Others approach the ending incrementally as part of the general rewriting process. Both of these are fine, but I've found that taking the ending alone and going step by step before to make sure the essentials are there often saves me time. Here's what I do:
Clarity - The easiest way to assure that your ending is clear is to have someone else read it and tell you (without prompting) what happened. You can go through and dissect the elements of the ending and look for ambiguities in the language, but another pair of eyes is much more efficient. I like to have people who are likely to enjoy the work and who are unfamiliar with it take a look. I do have one caveat. I have seen many people in writers groups explain and defend an unclear ending. (They seem to imply that it's all there, and the reader is at fault.) There is no value to doing this. Just listen and fix it.
Resolution - Check for this by articulating the story question and its answer. This means actually writing out both. What is Rosebud (Citizen Kane)? A sled, but also a symbol of the loss that drove Kane. Will the rescue team save Private Ryan (Saving Private Ryan)? Yes, but only by sacrificing many men who also have hopes, dreams, and families.
Note that while your questions for these movies are probably the same, your answers might be different. Writing down more than one answer is fine. The point is that an answer can be given for the story question. And it should never be simply yes or no. You want "yes, but" or "yes, and" or "no, but" or "no, and."
Logic and fairness - This is less clear cut, but here are the questions I use to test my endings:
- Have I followed cause and effect? Do events emerge as a natural consequence of what came before?
- Have I been true to the "rules" of the story? Does what happened make sense (especially in fantasy) given the constraints of culture, science, history, and behavior?
- Do my characters continue to act "in character," making choices that are consistent or changing with sufficient and clear motivation?
- Did I provide the elements that come together for an ending early enough? Did I avoid introducing a deus ex machina (a rich relative, a gun hidden in drawer, a suicide note) that made the answer suddenly easy?
- Does the ending come from the actions and decisions of the protagonist rather than from someone else?
- Was the ending made possible by the changes to my character as he or she worked through obstacles? (If the character could have done what was necessary from page one, I have a story problem.)
Timing - This is the trickiest of all. Keeping a list of loose ends that matter for a reader and making sure these (and only these) are answered helps. Length matters, and a good rule of thumb is Act 3 is no more than 25% of the work. It is always good to see what happens if paragraphs and scenes are cut. I often find that I didn't want to finish, and cutting either works directly or points me toward more compact ways to present the ending.
I prefer abrupt endings, both in what I read and in the final versions of what I write. I hate codas and epilogues. I am irritated by a kiss that takes place after I already know the hero and heroine have found love. But most readers like to have a bow put on the story, to have what they discovered at the end confirmed. I recommended keeping it (or, in my case, adding it) and asking explicitly what readers and editors think. Just don't overdo it.
I work in this order, and I include all the ending (usually Act 3), not just the final scene. When I have gone through this process, I have some confidence that I have a good ending. It doesn't guarantee that the ending is great or even entertaining, but it should be satisfying.