Groove 3 is a great place to start with understanding orchestral stuff. Even if you join for only a month Groove 3 is worth it. They have several good video tutorials on orchestration, how to do it and how to mix it and more. There are also books available on Groove 3 (not that I read them).

I recommend “Orchestral Library Toolbox” which is very extensive not specific to any instrument product or genre. It covers many aspects and provides hours of details. Vienna Smart Orchestra Explained is another good video as are MIDI Orchestration Explained and Orchestral Mixing Explained. It should be possible to gain a good understanding if one has a look at these videos.

I found Orchestration way more complex than many other aspects of computer music. Even more complex than pedal steel (which is also complex IMHO). Whilst it is complex and a big learning curve Orchestral Instrumentation is a lot of fun and can keep you occupied for hours on end. (That is basically the reason I’ve had less time on these forums).

Have fun Tony

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