Determine the theme of your song. For example, if your girl left you for another boy, write a sad or bitter kiss-off song. If you are head-over-heels in love, write a romantic ode to the girl of your dreams.
Brainstorm lyrical subject matter. Think about the relationship you have with this girl, whether it be fictional or not. Consider inside jokes and references you and the girl share. For example, if you and the girl used to go see movies all the time, you can incorporate movie references into your lyrics, such as tickets and popcorn.
Write verse lyrics relating to the girl. Each verse should work together to tell a story. For example, your beginning verse could reveal how you met the girl and how beautiful she looked that day—detailing the style of clothing she was wearing or how long and flowing her hair might have been.
Write chorus lyrics relating to the girl. The chorus should express an emotion and sum up the theme of the song. For example, if your song is about a cheating girlfriend, your chorus lyrics might be "You're an empty-hearted girl."
Create a melody to the verse and chorus, depending on the song's subject matter. For example, if your song is romantic, sing it slow in ballad time. If your song is about saying good riddance to an ex-girlfriend, you could sing it in angry, fast-paced rock and roll fashion.