i guess in the same way kinda that other songwriting partnerships work.... just slightly more convoluted...
its the first time ive done it this way.. but for me i hear the song as i write it- hense it fits to my tune, but when i was parsing lyrics etc, i look for things like the meter (be it iambic quatramic or pentamic) and see if whts therefits into it,and other things like that... the kinda thing you learn from grade 7 music theory :S