I wouldn't say I ever been properly "trained" either. Most of what I do is trying to emulate something I've heard before. The best resource is of course scores, which you can find for free for most classical composers on imslp.org. Since you've mentioned Debussy as an inspiration, here's a link to his page: https://imslp.org/wiki/Category:Debussy%2C_Claude. Also, listen to as much music as you can, and when you find something you really like, get your hands on the score and study it like crazy to figure out how it works.
From a technique standpoint, I guess I tend to think of music in layers, like melody, countermelody, accompaniment, chords, bass etc. Try to break you're music down into these layers, and experiment with using each instrument in the different layers. When you study scores, try to analyze them in this manner as well by imagining what layer the composer intended for them.