Someone gives you lyrics, and it's your job to write the music. In real life, many songs are written this way. Think of the long, productive collaboration between Bernie Taupin (lyrics) and Elton John (music), for example.
If you find yourself in this songwriting situation, where do you start? A great deal depends on your relationship with the lyricist.
Sometimes the lyricist has no attachment to their work and is just glad to play a part in your musical efforts. In other cases, the lyricist might be maniacally invested in every nuance and syllable. It's better to be clear about their attitude before you start composing. In any case, it's good to ask the lyricist if they had any bits of musical ideas, and if they have a particular musical style or mood in mind.
Finally, when you're alone with the lyrics:
- First, figure out how to fit the lyrics into a conventional song structure, usually with verses, choruses, and a bridge. If the lyricist hasn't provided a chorus, can you invent one by picking a phrase that sums up the song and repeating it a few times?
- What is the emotional tone of the lyrics? What kind of feel and mood do you want to create with the music?
- Start with the most central part of the song, usually the chorus, and figure out a mood, a tempo, and a rhythm that makes that part of the song work.
- Work out specific notes and chords for the chorus.
- Then rough in the music for the rest of the song.