Sitting down to write can be like taking a hike in the woods. You can look for the designated trails - the upper loop, the lower valley, maybe the creekside path to see waterfalls - or you can just strike out into the trees and let your feet set the direction. Writers have different ideas about the process of setting down words. Whether it's fiction or non-fiction, there is no prescribed way to "put pen to paper" or fingers to keyboard. Some like to construct elaborate outlines which detail characters and scenes; others simply sit before a blank page or screen and allow the story to take them on a journey. I don't think there is any science to the writing process because it is so subjective. After all, we're not dealing with an empirical construct here.
I like to write more from the seat-of-my-pants, so to speak - more in the vein of allowing the story to take the lead. But I don't work from a completely blank slate. I always let the idea for a story or book gestate awhile so I can consider every angle. Therefore, by the time I sit down to write, I have a pretty good idea about a general direction. Sometimes on the journey I will alter the direction a bit and find a new character or two, maybe a new plot point, but more often than not I just follow the lead of the story.