I'll be honest, I wing it.

I almost always wing it, unless an awesome idea pops in that absolutely HAS to be used, but I've really only had that kind of thing happen once. I have tried to make an outline for a story and make a formalized approach toward writing a story, and I really just didn't enjoy it in the slightest. Felt more like work than fun.

Honestly, when you wing it and just throw a lot of ideas and things on the table, tying up the loose ends is something that can wait until later. I also find that winging your story writing allows you to be more unpredictable and not feel like a train just chugging along a track you built for yourself, which can get derailed if suddenly you realize you forgot to have a means to connect point E to F or J to K. Writing by winging it means I get enjoy it more because I get to write whatever I feel like writing about at that particular moment, just so as long as it could potentially fit in the story's context (and writing fantasy allows you to do pretty much anything).

As for finishing such a story, you just need to eventually have most of the loose ends get tied together, resolve the conflicts, and have some kind of conclusion come out of it. Chances are good if you wing it, the reader might not know what to expect out of your ending! You would have taken your inspiration for a story from many different sources rather than the limited few you used on the days you planned it out.

I used to make mazes for my mom just for fun, and honestly, winging a story is a lot like that. You create a lot of open paths that lead in varying directions, and every now and then you close one off, open a few more, close a few others, fill in the empty spaces with stuff, and eventually close off all open paths until only one gets to the end. Characters in a story should be in a situation where not every path seems to take them in the right direction and they need to go back or work in some other solution. Sometimes it may feel like they're going the right way, and then bam, they realize they took a wrong turn but there's still some open possibility of getting back toward going the right way again, even if it looks like hope is fading. You should actually want to have your characters get hurt, lost, confused, saddened, and into all kinds of conflicts along the way. It makes them much more likable and interesting.

I understand this method isn't for everyone, but I enjoy it the most and it allows me to just enjoy the story at my own pace while great ideas come to me rather than draw a wavy line first and build the maze around that.