Well Hector, I'm a composer who's written for orchestra several times, and I'm happy to tell you how composers typically wrangle that beast.
There are 3 primary ways composers work, depending on their preference:
- Piano Score
- Short Score
- Open Score
Piano Score – when a composer writes everything on a grand staff (like you would for piano). Often composers will add little indications to show which parts should go with which instruments (e.g. "fl." for "flute" and then a connecting line to a run of 8th notes). Many composers are pianists, so these is an easy way to work for them. After the piano score is done, the composer will orchestrate the music, assigning different parts of the music to different orchestral instruments. (I'll explain orchestration a little more below).
Short Score – a short score is somewhere between a piano score and a full score. A short score will typically have the grand staff as before, but might include additional staves of fully notated music. So for example, if a composer knew they wanted lots of big brass chords but little tiny fast fluttery woodwind sounds, they might elect to put the chords on the grand staff and use 3-6 additional separate staves for the woodwinds. Again, once the music is done, orchestration.
Open Score – also known as "Full Score", this is when the composer writes the music directly on to a full sized orchestral score. Instead of 2 staves like in a piano score or 5-10 in a short short score, there might be anywhere from 24-60 staves depending on the size of the orchestra and the type of piece the composer is creating. Once the composer is done the music here, orchestration is already done (however, if they wanted to play a reduction for someone, they'd have some work cut out for them!)
There are pros/cons to each of these methods. I won't go through all of them as that would take awhile, but I'll give a couple examples: piano score pro: quick and intuitive / con: the most orchestration work. open score pro: allows the composer to instantly write for any instrument / con: if the composer is not disciplined, a score can become really clouded, unfocused, and disorganized. In the end, the manner in which composers write depends on personal preference.
Copying & Orchestration – if a composer is not well-known, prestigious, or established, often they will copy (make the score and parts) and orchestrate their own music. If they are those three things I mentioned, they'll often outsource that work to less established composers. This workflow is very common in movie scoring: in fact, there are companies dedicated to "music preparation" that prepares the written music for the orchestra, and movies often have teams of orchestrators to translate the composer's music into a full score.
Putting it in people's hands – orchestras are big and very expensive. For that reason, any time an orchestra plays or reads music is a very special thing. Professional orchestras might rehearse a piece 1-3 times before a performance. A conductor will ALWAYS spend more time on old, dead guys (like Beethoven and Mozart) than someone new and living. This is sad and should be reversed, but I digress.
If it's part of a commission (a financial agreement between the composer and commissioner), then workshops are typically included and outlined in the commissioning agreement; the people who pay for the piece want to make sure it's good, and they'll often schedule workshops throughout the processes, sometimes 1 in the middle and perhaps 1 toward the end before final delivery. During the first workshop, they obviously don't expect the entire piece. For that workshop, the composer should endeavor to try out as many different ideas / sounds as conceivably possible. (I once sat in on a workshop in which a composer didn't have any part of a "piece" but spent the entire 30 minutes having the orchestra play 20-second gestures. This is an excellent use of a first workshop.) However, the 2nd or final workshop needs to have a completed draft. That workshop should be for tweaking and adjusting the final product before delivery.
Almost all commissions / project have a specific timeline, so the it's the composer's job to make sure that they stick to that timeline as well as the guidelines of the commission.
Computer simulations are "okay" but mediocre at best as to be truly effective they need to rely on what we call MIDI Programming to make them sound really realistic, and this is a very time-intensive process, and again, people specialize in doing it (especially in film, where they can make up to $400/hr!) In the end, the absolute best thing is to always hear the music played by live, human performers. It gives the composer the truest reflection and sense of the sound.
Last point – and this is really important: composers are not mystical wizards. Yes, the final product sounds really impressive, but like most everything else humans do, writing for orchestra comes in layers. General ideas and sections take place, and from there, more tweaking and details emerge. Composers are always weighing the music they hear versus what's in their head, and they try and make the two match up as closely as possible.
Hope that helps.