How my dad describes charcoal drawing, "...like a camera coming into focus slowly... large general soft shapes, lights and darks, and eventually crisp details and accents that make it a polished piece." Same as writing, same as painting, a couple little bits stand out or are dictated, and the rest must be broadly and organically laid down and then later built upon.
After finding the key, he hums along with the song where he feels its right, maybe spitting out a couple of words or sounds that pop into his head, and just getting a general mood and feeling from the song. The tricky lines and words come later, working in reverse would create a constipated, shoe-horned end product that would lack the free spirited work that is in tune with the track its layed over, when the right process is followed. That's what I think all artists would tell you.
Commercial art tends to put bullet points and limitations on the end product, but if you keep this process in mind, you can flow around these obstacles and accomplish the task creatively while adhering to the guidelines. That's IF you remember and practice the process.