Servus Christof,
Here comes the long version! ;-)
1. Turn OFF all "real" FX like reverb, early reflections, delays and so on. Make notes how _you_ think they should sound (long? short? far? near? "classical"? bright? ...) and hand them over to the mixing engineer.
2. If you deliver stems in the sense of "ensemble groups", make sure that all musically relevant balances and volume rides are recorded within the stem. IOW: Leave volume automation ON! No mixing engineer will know exactly how you expect the single elements of your ensembles to interact (neither will s/he have the time to figure out). For example: Your VI-1-sections is made of Appassionata Violins and Solo Violin. Put them together on one stem.
3. Each stem should have proper gain, i.e. peaks around -1 to -10 dBFS (in 24 bit, of course). If there are instruments which have a much lower gain throughout the whole piece, bring them up to a value similar to the ones mentioned before, but make another note for the mixing engineer. Except for sound-design reasons, don't use dynamic processing like compressors or limiters.
4. Apart from instrument- or ensemble-inherent pannings, the stems should be centered (read: full stereo, NOT mono). Make notes how you think the single parts of your piece are meant to be used, if you have a concept in mind which isn't necessarily a "classical" solution (e.g. VI-1 left, VI-2 right, CB center, harp center behind the CB, etc.). Hand the notes over with the other information you gathered already.
5. Typically, your stems could look like this:
VI-1
VI-2
VLA
VC
CB
Horn Ens
Brass Ens 1
Brass Ens 2
Wood Solo 1
Wood Solo 2
....
Timpany
Harp
....
Synth FX Stereo
Synth Percussion
.... and so on.
One important point is to keep dedicated solo-parts separated from "ordinary" ensemble parts, to give the mixing engineer an obvious reason to take special care for them ;-)
6. Don't forget to make detailed notes about the format (e.g. "24 bit / 44.1 kHz / interleaved stereo / *.WAV"), the number of files and a version-number on the storage medium itself (Data-DVD, FTP-Folder, whatever). Give each single file a version-number and the creation-date _within the file-name_, to make sure that the proper files are used in case of an update at a later date. Give clear indication that all files share the same starting point (the preferred case), or make sure to provide a proper Standard-MIDI-File with a tempo-map if this is not the case (definitely _not_ preferred!).
7. Supply a rough-mix with the same starting-point like the stems, for clear reference what has to go where.
This should take care for most possible pitfalls. All the best!
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/Dietz - Vienna Symphonic Library