In many song collaborations there is usually a lyricist and a music composer. Sometimes the lyricist is the vocalist. This person may also be the performing artist.
When these two people begin a relationship it is with their individual talents in mind. They pool their resources and begin composing a song.
In my situation I am usually both the music composer and producer. I create the music that will be performed by the vocalist.
I then pair with either the artist or an outside lyricist who will work with me to create a complete song.
The pre-production of the song is vital and the criteria has to be established from the onset of the song construction.
A few things to consider are:
1) Who is the song being written for?
2) Is the vocalist male or female?
3) What is the vocalist's range?
4) What is a an effective tempo for the song to be both engaging and conducive to a great vocal performance?
Once the ground work has been laid, then the two composers can begin constructing the song.
I generally work best when left to my own devices and build the song in the studio. I meticulously record every part. If the song requires instruments that I can't perform, then I will hire musicians.
Each time I record a draft of the song I email the idea to my parter for evaluation. If we are on the same page with the direction of the song, then I will continue to produce it.
The arrangement of the parts will go through several different incarnations until we decide on the most fluid version. The best is usually the one that moves freely without any distractions or clutter.
Every draft is then sent to my partner for approval and creative input.
Once we've established some semblance of a song I'll bring the vocalist in to perform the parts. By this point the song has taken a definite shape and many of the performances may wind up in the final version of the song.