Play the song to students. Give them some time and resources (internet access, dictionaries) to go through the lyrics in pairs or groups of 3.
For the purpose of this lesson, it is important they do this alone without too much teacher interference. In my experience, many want to know what it is about as the song is all over the place at the moment.
Ask students to take a stand on whether this song is promoting good self-image of women, whatever their size or not and write a standpoint in which they explain arguments pro or contra.
Pairs or groups first physically take a position on this question by going to one or the other side of the classroom and then read out their arguments for their decisions.
Students “correct” the lyrics of the song so that it really promotes self-esteem. Depending on the resources, they can either write their versions on paper and post them on the wall or even produce a video of the better versions.
Here is one good example of an argument and the corrected version of the song:
Note: if you are interested, there is a lot of controversy about this song and many people have bothered to write about it. See for example: