Choir: This is probably the hardest part. You have to have harmony for the choir. A standard method for writing harmony is to have the Soprano sing the lead; the Alto would be a 4th below the Soprano; the Baritone would be an octave below the Soprano; the Tenor would be a 3rd above the Baritone; and if you want a Bass, put him at the root of the chord below the Baritone. All notes have to be analyzed to make sure that they fit in the key. A 4th could easily have to be adjusted to a 3rd or a 5th depending on the song. Obviously if you want to get creative, you don't have to follow this guide. As long as the vocalists stay within the chord and within their range, it doesn't matter what the intervals are.