I had a general idea of my start, middle, and finish. I'd gather inspiration from my motivation (as in, I'd be writing and writing and would allow myself to go for as long as I could) orrrr I'd gather inspiration from songs and reading.
I honestly know how I want to end part one of my story, but I only have a general idea of how I'm going to go about part two. I have a lot of research to do. Research has been a huge part of expanding my story and plot into more than I could have anticipated it to ever be.
Sometimes it just takes falling into the mindset of the character who's POV you're writing from and just letting your fingers type. You can always go back and re-read and edit what you have done, but letting your mind wander and create a story, through a character, can get you far.