I tend to write words to existing tunes - most frequently extra verses to existing songs although I have written tunes to poems and both tunes and lyrics.

I think it's probably easiest to start with the odd verse and chorus, work out the details of the tune - you've probably got a rough idea of it to start with from the metre you've chosen. Then you then write the extra verses. I find I need the tune to make sure that the verses fit well.