I tend not to write original pieces as much as reimaginings/adaptations of public domain works. (My current song cycle about mental health, the autism spectrum, and the diversity of minds in the 21st century notwithstanding.) Here are the work processes for the two shows I worked on this year:
PRISONER OF ZENDA: I was writing book and lyrics, so I revisited the source material, taking copious notes- story outline, defining character traits, quotes I wanted to use or reference. I then wrote a five-page outline, building the show structure, locations, act break and places I knew we would need a song. I then went through, writing scene by scene from top to bottom, drafting the songs as I came to them, removing song cues that were no longer necessary, then adding new songs where I deemed fit. Day by day I sent new sides to the composer, who worked with me on what works and what doesn't, making changes alongside me as he composes. In this particular case, however, if the project has a leader, it's me; the composer is writing to what I create, as much as writing WITH me.
CABINET OF DOCTOR CALIGARI: This is a somewhat different situation, as I am still librettist, but am sharing composition and some lyric duties with a co-composer. In this case, I have drafted a "working draft" libretto, with potential song cues indicated, along with a "writer's note" to both him and myself describing what I feel this moment needs, sometimes including a dummy lyric as a placeholder if it's a musical sequence coming in and out of dialogue to a specific end. Then, he or I will write a lyric, and he or I will write music, depending on the specifics of the scene and how it speaks to us.