I could say get off the piano and write from a desk. Pencil in hand, manuscript paper in front and listen. Then write what your hear, feel and sense. For me, the tune or rhythm or both, sometimes words come into my head and I spend a little bit of time working out the gist of what it is. I sketch it out on manuscript then test it on an instrument. I correct my writing, if needed, or allow the differences to remain. It’s only when I put it in context with whatever project I may be working on that the real writing begins. Often it’s a stream of consciousness with the notes just flowing on or I hear other instruments coming in and adding a counterpoint or taking over. It’s always free flowing at first. Then I put it all together and edit some more. I tend not to rely on hearing the sounds until this time. My imagination gets me through most of the time.
Have a listen to some of my work on jonathanmorrison.bandcamp.com check out the work on the album Nest (Tracks 3,4 and 5 are the orchestral pieces - the others are for piano and rock band)
all the best
Jonathan