Dunno about good, but I have enough theoretical knowledge. I could score out my compositions if I had players to play them, but it would be a lot of work and I'd be really slow, and constantly referring to my text books to get it right.
When I first started making instrumental music, I did a lot of learning and reading, specifically orchestration. I can highly recommend Walter Piston's book 'Orchestration' as a complete guide. Everything else is just picking up what you can where you can.
As far as creating the pieces goes, I wrote them as I recorded them, playing each part into my DAW on my midi keyboard.
I hope some of that helps
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