Fifteen thousand words is a really good start. I don’t know how many words I wrote my first time around but it was approximately 14 chapters, as I divided it at the time. Then I didn’t know what to do or where to go next. I had a friend read it, one chapter at a time, but she didn’t get far till she said she can’t handle it. She couldn’t tell me why not. It just sat on my computer until my friend found out about a local writing group.
These other writers asked me questions re how did this happen, why this, how that. In response, I wrote chapters leading up to what I had already written. I added some more to the end, too. Some new ideas were added to the middle. By now, I probably have enough scenes for more than one book but I still have to organize it into a coherent whole. I’m working on that right now.
I guess the answer to your question is: I skip around a lot. Because my book is not yet done, I don’t know what will end up between covers and what won’t. Some writers are really good at planning, but I’m the opposite. I’d rather end up with two or three times the amount required so that I can cut as needed to make it fit.