Reddit mentions of The Study of Orchestration 4E International Student Edition with Access Card

Hey there - it depends on what sort of you music you want to write and how much you already know!

A good place to start is ... reading! :O

But seriously, books like Samuel Adler's ["Study of Orchestration"] (https://www.amazon.co.uk/Study-Orchestration-Samuel-Adler/dp/0393283739/ref=sr_1_1?keywords=study+of+orchestration&qid=1557587743&s=gateway&sr=8-1) is perfect for learning about the common orchestral instruments, and other books like Elaine Gould's ["Behind Bars"] (https://www.amazon.co.uk/Behind-Bars-Definitive-Guide-Notation/dp/0571514561/ref=sr_1_2?keywords=behind+bars&qid=1557587760&s=gateway&sr=8-2) is an excellent guide for creating professional scores.

Outside of reading, listening to orchestral music of the 20th century is also a great learning tool, particularly composers like Stravinsky, Holst, Britten, Tippett, etc.

And if you can read music, following the scores/sheet music as you listen is even more useful (you can find a lot of free scores on [IMSLP] (https://imslp.org/)). If you have time, follow one instrument at a time, see how the instrument is being used, what it typically plays ("idiomatic" writing), the general ranges and techniques, etc.

And then just have a go at it - it can't be learnt overnight (heck, I've been doing it for nearly 9 years now!), but if you keep chipping away at it every now and then, progress will start to show!

^^^/WallOfText