I don’t always start with an ending in mind, though I usually have a vague idea of where the story is going (I’m half plotter, half pantser).
I usually have the hook/premise along with well fleshed-out characters and a few key scenes. Once I’ve gotten to the first pinchpoint I usually know more or less how I want the story to end, but don’t necessarily know how I’m going to get there, or what the climax scene is going to be.
By the time I’m halfway through I’ll have plotted the rest of the book, sometimes down to pretty fine detail. Throughout the first half, a lot of changes to the plot and to the way the book ends will have popped into my head, often making the beginning of the book in dire need of edits by the time I get to the end. (No rest for the wicked, amirite?)
The only exception to that is the next book on my queue, which is the last one in a four-book series. That thing is plotted out almost scene-by-scene at this point – only because I’ve been thinking about it for so darn long.
The thing I struggle most with is just being okay with not knowing for a little while as I wait for the right ending / the right inspiration / the right idea to snap into place. It takes a little time for my ideas to marinate, so sometimes I get impatient and try to plot things out before they’re ready and I wind up with something flat or rushed that needs to be scrapped and rewritten. That was the case with book three: I finished and was in revisions when I wound up scrapping and rewriting almost the whole final third of the book.
I know it when I feel a good idea for a plotline resolution hit me, though. I practically jump out of my chair shouting “eureka!”