Planning a Composition
In spite of many composition teachers and professors recommending to plan out a composition properly beforehand and making sketches to create a proper structure and drama for a piece, letting a composition develop out of itself might be a valid approach to writing music.
There are often interesting ideas and possibilities opening up while you’re working on something which you would need to often ignore if you slavishly followed a sketch or plan. My personal process often is based on having a structural plan beforehand knowing how I want the piece to feel like and having decided on the general sound but many of the decisions of taking specific melodic, harmonic end especially orchestration paths happen on the way.
The more experience you have in composing/orchestrating the better you will be able to still keep an overview over the big structure of your piece in spite of giving in on small impulses during the working process. Of course, when writing music to picture, you need to plan ahead a little more so things will fall into place later on.