For me it varies. I mostly have a rough outline in my head with a few clips being definitely planned out. Though it's always possible that I will change my mind while vidding. Sometimes I stumble across a new perfect clip while clipping and sometimes my original choice does not really work after all.

There are also some vids that I want to make for years and it takes me forever to get started and then there are those where I suddenly have an idea and a day later I have a first draft.

Sometimes I just want a certain vid to exist but I have no idea how to do it and in these cases I have to start putting stuff on the timeline and see if it leads anywhere.