alrighty, I'm trying to come up with a finale for my 17th chapter, but I don't know how to plot it out. I have a general Idea of what's going in it, I just don't know how to map it out or put it into words. any advise?
Write the finale with your general idea in mind. Ideas will occur to you as you write. You can make notes on the plot at this point, keep writing or start again using what you have learned. The process of actually trying to write your general idea, i.e. making it specific, will help you to make it something you can really use.
I am a believer of brainstorming, talking it out with someone or multiple trusted writers. Nothing to lose by bouncing ideas off others, and sometimes just the process of "thinking out loud" will deliver the answer you are seeking.
It'll come with time, no need to worry. One thing you could do is view movies/books that are similar to your story and see how they end; you'll get some ideas.
If you know how you want it to end, and you know where you are then connect the dots. In other words... If Frodo just barely got to Mount Doom and the ending is, he destroys the ring there, then write about that. If where you CURRENTLY are in the story is too far away from your ending that it would seem rushed and an abrupt ending, you may want to consider adding on 1 or 2 more chapters. The ending should tie together all of your ends. The final chapter should be where all your visions of the ending are written out. If you envisioned your ending where the hero comes back just in time to save the day, gets the girl, and rides off into the sunset, then expand on those ideas.
Maybe I am an oddball in this, but I personally think the ending is the easiest. I see the ending as the end goal and objective you have been writing the story towards the whole time. By time you get to the final chapter, you should have a pretty clear idea of how you want the story to end and what is going to happen.
I would go back and look through the story, read it from a customer's point of view. Let your own story take you on a trip through the world you just created. Don't think about editing, or trying to change things. Just read it as you intended. That should give you a better idea of where you are in the story and it could very well be that you need to add more before you end it, or maybe you realize that the story should have ended a chapter ago instead.
I know this doesn't exactly explain how to come up with the right words, but it should at least give you a good start by reading through your own story to see if you find any inspirations or lose ends to write about.
Write question about the story and answer them yourself.
Could David know what was going to happen? Yes, because: No, because: Did Sarah really cause the accident? If no, why did she not tell?
It's also a method to discover possible flows or plotholes in your story.