This has a lot to do with what sort of music you are playing, and what sort of part you want from your instrumentalists. Jazz and funk horn players, for instance, generally can create their own parts by working from a chord chart. And, if you want solos, that generally is pretty easy.

Other than that, you'll have to write out a pretty fair outline of what you want them to do, even if it is only in concert pitch, or be ready to run a very long rehearsal--