Or usually I have music, then I have lyrics I have also written and I basically do kinda the same thing, but I already have the lyrics, its just trying to fit the lyrics into the song. Also that means you may have to modify your music a bit or a lot depending. This is actually a good idea to me because I will usually have all the musical hooks and everything written with verses then when I try to fit lyrics in I just leave the hooks and basically write corresponding verses (musical ones that ake the lyrics I have) and then combine them both so I have good music sections then lyrical sections that are also good and when the lyrics are going the music generally backs off a bit. Sometimes it works that lyrics you have and music just go together perfectly.
Think about meaning when writing music for lyrics and put emphasis (with music) where you think it should go.
There aren't any real rules that I know of I basically just go for it and sometimes it works and other times it doesn't. I don;t have a ton of experience so I am kinda where you are, those are just my ideas on how to do it.
