Great ideas in this thread! Thanks everyone!
Personally, I like to listen to my favorite composers and write down a score or put it into DAW. This is a great way to learn harmonic rules and see how professionals build up tension. If someone better did it before you there is no reason to reinvent the wheel
BTW. Reading books like: Rimsky-Korsakov "Principles Of Orchestration" may be also helpful.
Edit: Sorry, made some mistakes ;)