First I start the lesson by showing the music video for blank space. Then after it has finished playing I have the class guess what the song is about. After we finished that discussion I then hand out the blank space lyric sheets and play the music video two more times so that kids can fill in the missing lyrics. Then we dicuss the real lyrics and ask their opinions on the relationship presented in the song. Afterward we do a relationship worksheet where in groups of four students rank the factors for a sucessful relationship  from 1 to 16.
The relatonship worksheet is found at http://bogglesworldesl.com/loverelationships.htm . But Ive also attached a copy that I downloaded and the blank space lyric sheet.