Practice Working With Limitations
As a learning orchestrator and composer you should practice a lot to work with limitations, especially if you want to eventually work with real musicians.
While it is great to know how to write for a huge orchestra with all instruments anyone can imagine, reality and budget constraints regularly force us to cut corners and still deliver something impressive. Additionally for a learning composer, it is essential to be forced to not hide behind impressive orchestration. A good learning effect is always to ask yourself whether your piece would still be good if it was played only on the piano.
But essentially, in order to be prepared for the real world, try to write a piece with limits. Can you write effectively for strings only? What would you do if you only had a few brass players and still would need to write something epic and grand? Can you handle woodwinds alone? What if you only had a string quartet? In fact, orchestrating for a large line-up is actually easier than orchestrating for a reduced line-up. It is much trickier if you have to ask yourself things like “I only have 2 horns but I need a harmonic support with a full triad, where do I put the third note? Or do I actually need that third note?”
The more flexible you are with that, the better. Even on big budget productions, directors or your own conceptual idea sometimes limit the sound to a specific line-up and you don’t want to stand there not knowing how to handle it.