How do you write a songfic for fanfiction?

I haven't exactly READ a songfic before, but I'm thinking of writing one. It's a cute Reader X Swiss fluff (hetalia). the song I'm thinking of using is "Best Friend's Brother" because the reader is friends with Lilie and has a crush on Vash. But since Vash isn't really Lilie's brother, Lilie has a crush on him. Hence the lyrics "I don't want her to know." I was attacked by this plot bunny and can't get him off. But what are the key points in songfics??????