BW,
The advice you just got from Paul and Dave is the foundation of what separates average composers from great ones. Dave mentioned the lost art of part writing.
The single most important skill to have. Today's music, and that's a definition by opinion more than an accepted fact, is missing in a sizable percentage the skill and craft of part writing.
I won't speak for Paul or Dave, but in my days as a music student part writing was an ongoing lesson, and i scribbled hundreds of 32 bar, 4 part attempts, tearing up, changing, editing in the search for good, reliable part writing. It was an essential skill that i rely on more than ever today.
If there is one skill that, with determinaton and practise, will give you a better chance of getting your ideas into a musically interesting piece, it's part writing.
Write it down first. Then input it, then listen, and hear where it doesn't work.
And again. And again. Until you get past the conscious effort of remembering what goes where, and instinct takes over, and you can invent and playback in your head, 4 part inventions that work, and make sense.
Dave also mentioned transparency. As well as the important point he made about hearing everything work, you also get to learn just how different instrument combinations work or don't work together, and then the library of 'sound' ideas increases in your head.
If you approach part writing as an interesting challenge and an essential skill for now and the future, it will be easier to write piece after piece searching for the best result. Paul made a great point of keeping them short. 32 bars is easier to analyse than something 200 bars long. Start small, work up to the bigger stuff.
Write 4 part until you can do it in your sleep.
Then add an extra voice, and do that unitl it works.
Then another, and so on.
Before you know it, you'll be writing for full orchestra with confidence, variety, and interest, without the feeling of taking something on that's too big, and get overwhelmed.
I wish you the very best in this and the future. You've asked one of the most musically intelligent questions you can, (In my opinion.) and that bodes well for the future
Regards,
Alex.