If it's a song the students aren't familiar with, you could also ask them to predict what the song is about based on the words you've chosen. Next, put the students into pairs or groups and allow them time to compare and revise their lyrics. Circulate and assist as needed. Finally, once the students have most of the lyrics, play the song again so they can check their work. If it's a song the students really like, you can even play it a fourth time and have them all sing along.
Students write down the lyrics.
You play it again. Students write some more lyrics and compare. Eventually, you review it. You've gotta at least look like you've prepared.