If something "pops" into your head, it's probably a good idea. Think about a theme and let the lines fall around it. What I mean is that good songs write themselves. You're just holding the pen. And it doesn't always come at once. Some songs can take a few minutes; some a few weeks, etc....
A lot of stuff I've written began with lines that end up in the middle of the song, at the end, or maybe the first line. Keep a notebook and write EVERYTHING down 'cause the best ideas are gone very quickly after they come to you. Always trust you inspiration. You know how they say, "be yourself". It's like this with lyric writing. If an idea comes to you, it's yours, and that's what you must follow. And remember, it might only sound silly to YOU. I had a lot of experiences where I'm improvising with some guys 'cause we have to write something for someone, and I sing something that I think is silly, but the other guys say, "That's great. Let's use it".
Having the chord progression worked out definitely makes it easier. Then noodle around with a melody. Very often, you'll find words falling on the notes you sing.
Hope this helps. Good luck.