One thing to keep in mind, Some instruments are not tuned to Concert C.
Flutes,Horns, etc. are tuned to something else. Like... A Bb on a frugal horn is a C to us. So everything has to be transposed.
Most orchestral instruments aren't supposed to be playing chords, but rather notes, so that's how I would write a string piece.
If my song is in the key of A. I might have a I IV V I string interlude. (No matter how weird that may sound.) So I would take my chords, like A (A, C#, E) And have my instruments play those notes, respectively.
Another way is to write a melody on the lowest of your notes, and then turn those notes into chords by stacking thirds. (Think Figured Bassline.)
That's all I got to say.
