I'm not an expert in the field, but here's my $.02:
- if you have sheet music/tape from which you can read/listen the various parts, then all you have to do is read/listen carefully :wink:
- if, as I suspect, this is not your case, then you might want to give the sopranos the melody. Like on the piano, this should be the highest note sung by the choir. You can give altos and tenors parts that make up the harmony that you want to imply. In many cases, this means a third or a fifth down from the soprano part.
It's probably easier to demonstrate than to explain!