Well, that all depends on HOW you interpret your own music. In other words, how the instrumentals make you feel will help you come up with a theme for the lyrics. Then, using that theme, come up with phrases that match the rhythm of the music in your song. For instance, the majority of slow songs have a passionate, loving feel to the music. The lyrics and the music need to collaborate at once, otherwise, you'll have a song with a confusing feel to it. I hope this helps you out. If you need more information, just respond and I can go into further detail for ya.
