I usually have an idea before I start.
But I never have a defiant plan till I see the model. Not all looks work on every model and some models will inspire me.
But after I get that jolt of what I’m going to do, I start working. I no longer see what is in front of me as I start. But I see the final image in my head. And I work almost back words. Seeing the completed image and putting things in place till it matches.