What instruments have I actually studied? Most seriously, the classical guitar, of course. But before that I started on bass guitar, acoustic six-string and electric six-string. I also studied the flute for a year or so, though not formally. I studied voice for a year as well, with a good teacher. As I was saying in my answer to Nathan's comment, I have also taken piano lessons off and on for a year or so, but found it made my right hand feel very bad, so I had to stop. Oh, I've also fooled around a tiny bit with harmonica and spent a bit of time learning to play slide blues guitar, which involves not only the use of a slide, but also a completely different tuning. Wait, one more instrument: I learned to play the mandolin just for a performance of Don Giovanni. I have the suspicion that I could learn the basics of most instruments in fairly short order as I certainly understand the principles of learning music!
What has all this got to do with orchestration? As a guitarist, orchestration has always seemed to me to be one of those arcane fields of study, like the calculus or organic chemistry, that has little enough relevance to be worth the trouble. But my most recent musical revelation (epiphany?) as a composer is that I do not need to limit myself to composing just for guitar, solo and in ensemble. I'm either a composer, or I'm not and if I am, then I can compose for any instrument(s). So, on the occasion of a new orchestra forming here, I decided to write an overture for them and the experience was so exciting that I am going to write a lot more for non-guitar instruments. The guitar is extremely hard to write for and I was throwing away half of my musical ideas just because I couldn't figure out how to fit them on the guitar. Writing for orchestra was incredibly liberating!
But I did run into one problem: I chose string orchestra with tympani and a small group of winds--flute, oboe, trumpet and French horn. Can you see the problem? I didn't at first, but as I worked on the piece I realized that there was a constant balance problem between the flute especially and the brass instruments. So I finally replaced the trumpet and French horn with clarinet and bassoon. It gives me a proper bass line in the winds and solves the balance problem. Due to my background I am a lot less comfortable with brass instruments than I am with woodwinds.
So something I am going to do soon is write a brass quintet. The kinds of things you can do with that ensemble are the kinds of things I am really unfamiliar with as a player and composer (though not, of course, as a listener). It also occurs to me that NOT knowing the mechanics of an instrument may often mean that you might come up with entirely new and good kinds of music for that instruments. The example of Joaquin Rodrigo comes to mind. He knew very little about the mechanics of the guitar, but wrote spectacular, if difficult, music for it. On the other hand, Mauro Giuliani (1781 - 1829) was a consummate guitar virtuoso, but everything he wrote feels a bit like a technical exercise because of his familiarity with the mechanics of the instrument.
I have a lot to learn about orchestration, but it is not so much the arcana of the different instruments (what is the exact range of the ophicleide?) as it is the palette of timbres that they can produce in various combinations. And that I've been noticing and marvelling at for years. Shostakovich gets some remarkable orchestral effects.
Which brings me to my final point: I often read about composers writing a score on piano and then spending a lot of time--years in some cases--"orchestrating" the score. An example would be Gurre-Lieder by Arnold Schoenberg which was written, in voice and piano form, around 1900 but the orchestration of which was not finished until November 1911, by which time his style had undergone radical changes. I really can't see working that way. It is the instruments that come to mind first and foremost. If I am writing for chamber orchestra, then that is how it is written down from the very beginning. Is this simply because I am not a piano-player? Would it help me in any way to do sketches in abstraction, for keyboard and later 'clothe' these sketches in an orchestral garb? I don't know, I suppose I could try it to find out. But I tend to have musical ideas that are embodied in particular instrumental form from the beginning--for better or worse.
I should end this before it completely turns into an arcana that is irrelevant for most readers. Or is it too late already? Let's end with some music. This Symphony for Strings and Woodwinds, op 73a by Shostakovich is actually an arrangement by Rudoph Barshai from the String Quartet No. 3:
Here is the original: