For me, it depends. Sometimes the lyrics come first, sometimes the music. I just let things happen. If a lyric pops into my head, I will write it down either in my phone or on paper. Sometimes, I'll come up with a riff in the middle of the night when I can't sleep and I'm just strumming guitar to kill time until I get tired, and eventually, I refer back to old lyrics I've jotted down and it becomes part of the song. Sometimes they happen at the same time. It's not a formula for me.