The only way you are really going to learn orchestration is by a combination or “stacking” learning approaches.
You will need:
1. Work with real musicians: Knock on practice room doors, and throw an excerpt on a players stand.
You know all those ads you see around music school about “violin teacher available” (fill in the instrument name) take a lesson with them. Explain you are looking to learn orchestration, and you want one lesson. These are often grad students and at a high level of training. You will never learn the psychology of playing that particular instrument any other way.
2. Attend as many orchestral rehearsals as you can. Professional, University, Community orchestras etc. It does not matter. Just go there often. Find out if you can sit in and listen. Try and find out before what pieces will be rehearsed and get those scores and take them to the rehearsal.
Keep a journal and write down any insights you have.
3. Get one on one training with an accomplished orchestration teacher. Really, get a fucking teacher.
Gut check time on how serious you are about wanting to learn.
4. Join an orchestration study group like the AFM get a few people who you can tolerate and have a score study group.
5. Write, have it performed with real musicians, then repeat, repeat, repeat, repeat, repeat, repeat, repeat, repeat, repeat, repeat.
Orchestration is not something that is separate to the composition. Orchestration is composition.
It is a novice idea otherwise. The context/genre of what you are writing cannot be overlooked, as well as other formal aspects. Don’t get me wrong…. I know plenty of people do, but why concern ourselves with them ?
Both Stravinsky and Beethoven were tremendous orchestrators. Each has their own fingerprint, and the degree to which they integrated orchestration into the composition means you cannot simply surgically remove one. You could take the opening of Beethoven’s 5th and arrange it for Bassoon octet, and I am sure some novel moments will result, but it is not the same affect. Another test would be to take the famous Psycho shower scene and play it on piano. Not the same.