Ooooh good question!
I’m afraid I don’t have a simple answer because honestly – it depends.
When I write a long story then I usually write a scene by scene outline before I start writing. This is because if it’s novel length I’m more than likely going to get confused and write myself into a corner. The exception to this is Painting Layers of Love because I’m experimentally writing that with prompts and as responses to people’s comments. I’m letting it develop organically and letting the characters drive more so than usual.
With all stories (short or long) I have clear images in my head for parts of the story. I usually have a jumping off point and I usually have an ending – it’s getting from A to B that I have to work out. Sometimes I just start typing and see what happens, sometimes I write up plans and essentially type out the entire story as a long summary. Quite often it’s somewhere in between. What I do depends on how inspired I am when I get the idea and whether I can start writing immediately, or if I need to put the idea on ice for a little while.
I usually have a fair idea of structure. I definitely fall down more on the side of planner than pantser but I’ve grown to appreciate a mix of the methods. I use what’s appropriate for the story and the situation. Novels I tend to plan to death but fanfic I’m more loose with and how many notes I have depends on the story. Sometimes this means I get stuck. I know where I need to be sometimes but not exactly how to get there. That’s when I talk with some amazing fic friends who have never let me down yet. I’m so lucky to have them!