For me it’s start with an idea of a character or a scene. Depending on how well formed the idea is I might start writing straight away or if I don’t I will stick it in my notebook for future use.
I often don’t know the end before I start writing. For example, I worked on a longer flash last month and for a few weeks I didn’t know how it would finish until the idea came recently so I went back and finished it.
I can’t think that I’ve every waited until I had a fully formed/planned idea before I started writing.
I appreciate that my process would drive most people mad, but seems to work for me (so far).