I haven't done an AMA haha, thanks for the interest :)
I rarely have a complete idea set out in my mind when I start writing, though I try to think of a vague, general plotline before I start. I like to start by just trying to think of an interesting intro paragraph.
I usually have a feeling for when I've written a interesting story when I finish it, so then I'll take more time to go back and edit and perhaps add to it before I post it. I often post something and then add little bits to it later though (or make big changes). But people's reactions are surprising. Sometimes a story will take off that I didn't expect to get responses, or something will flounder that I loved writing.
Oh, and I use this to write actually - http://www.zenpen.io, and then copy the story over :)