for me writing is a lightning bolt moment. these do happen more if you write every day and you have habits, but its still just a bolt from the blue. for me, it starts with a character name. some weird part of me can picture a whole person from a name.
of course, sometimes it starts with a scene. this is most common if i have not been writing for a while. the best way to get writing again (for me, anyhow) is to set a scene, to jabber about a storm, or a diner, or who the fuck knows. the canary yellow first page of WAR DANCES that vibrates with each pull of a tension headache. just whatever.
very rarely do i start a blank page with a fully formed idea.