I usually write all the music first but I think every song is different and should be giving special treatment. But usually I finish the music, record it, find the vocal melody (may take several tries to find the best melody possible), then I write 5 drafts of lyrics, pick the best one then tailor it to the melody and make sure it sounds good being sung in that melody. I think its very key to be picky during this stage and write the best lyrics you can. That's just what I do. Hopefully this helps.