I often come to the conclusion that keyboard players, like myself, are better equiped, knowledge wise, to compose orchestrated pieces using sample libraries. Now understand, I'm not trying to be arrogant here I'm just speaking in purely general terms. I'm sure there are string or wind or whatever players out there who could probably compose circles around me but, generally, keyboard players have it a little easier. Why? Because we learned the entire musical range and the complexity of chord structures. It goes beyond just simple triads as you are only limited by the number of fingers (and sometimes toes) you can put on the keys. Someone who plays say the clarinet, on the other hand, was trained in only one musical range and probably has a limited understanding of how chords work. Again, I'm just speaking generally here and I'm not trying to belittle clarinet players. The point being, if you're not a keyboard player you might be at a disadvantage right off the bat.
Having said that, all of my works begin as piano arrangements. For me, if it sounds fine on the keys then it will sound fine orchestrated. However, it's easier said than done. Now comes the hard part, for me anyway, arranging and orchestrating. Do I want to keep it a chamber piece, something bigger, something really big etc. In the beginning, I would print my piano arrangement out and that became my roadmap. I would play and/or program the piece in my DAW and that MIDI track became my template for everything else in the piece. I connected the MIDI track to a simple piano patch just to make sure that the composition and arrangement is how I want it.
Now, let's say I want to start with the 1st violins. I would copy my template track and paste it in the 1st violin MIDI track. Then I would go in and delete the notes that I didn't want the 1st violins to play. Next I would assign keyswitches. At this point, you have a choice, you could record the first violins and bounce the track or you can move on to the rest of the string section. You don't have to worry about the voicing because you already took care of that in the template. So go to the 2nd violins, copy and paste the template into the 2nd violin MIDI track and erase the notes you don't want the 2nd violins to play. Then go on to the violas, celli, DB, etc. You may have to change your octave range here and there but that's easy just highlight and transpose. As a technical sidenote, do not connect VSL to the MIDI track(s) until the track is ready to record otherwise VSL get confused and the engine shuts down. At least it does in Sonar I don't know about logic.
Let's say you want to do something that's not in the template like add a woodwind arpeggio or something and you're not sure of the voicing. Well, that's where the paper score comes in handy. You just figure out what the chord is for that measure or whatever and you base your arpeggio on that chord.
Another thing I like to do with the template or master MIDI track is create counterpoint that I didn't know I had. Let's say you have some four part harmony playing. You can delete everything except the soprano notes here, the alto notes there. Maybe the bass in one spot here and the tener in another spot there, then transpose everything to the same octave in the range of the instrument you're editing and there you have it. Instant counterpoint. It isn't perfect. You might have to make some adjustments here and there and the results may not be necessarily desired but it works.
I don't mean to go on and on but this is generally how I've worked since I purchased my VSL libraries. Only recently have I tried to put myself in the mind of a string player instead of a keyboradist and compose in the string realm. The result was the Chamber String piece, "Child's Play" which is in my sig line. A very challanging piece for me as I found it difficult not to go back to the keyboard for harmonizing. Old habits are hard to break.
Don't despair Denis the more you work with it the better you'll get