Should I write lyrics for a song we're writing before we write the guitar/drum/bass parts?
I've this song two friends and I wrote and I agreed to put some lyrics to it but i really couldn't because I could not fit any theme or words to the music. So for me it only works like you said.
My band has probably 4 ways that songs are born:

1. Me and the other guitarist write a chord progression/riff etc, then I write the vocal melody and the lyrics
2. I come up with a melody and put it to a chord progression.
3. I come up with a lyric and put it to a melody
4. We jam something and like it, then turn it into a song.

If you limit yourself to only one way of writing, I think you'll miss writing some songs you like and that might be a little different from what you'd usually do.
But at the same time, you might need to work out a specific way that just works for you. Theres no right way or wrong way, just write and experiment and see what works for you.
My band has different ways. As guy_tebache said, don't limit yourself. I write lyrics, than come the drums and bass parts. I have a slight idea how the song should sound, but I rely on bass, and sing lyrics over the bass (sometimes it takes really too much time, but I always get it right) after that I come up with a riff that fits over drums, bass and that kinda sound "right" for the song if you know what I mean. That's how we usually make our songs, but sometimes they just pop-up during jam-sessions, even lyrics come naturally. You sing it like the lyrics were always there in that music, it's almost scary how that works out.
thats really not a good question to ask. as has already been stated, there are numerous ways to write a song, and theyre all personal preference really.

personally, i almost always write them simultaneously. i hear the song (a verse, chorus, or both) in my head, and then i simply "notate" the music and write the lyrics. then theres little workings to complete it or change it up slightly. ive always found it difficult to separate the two processes, especially the more complex the music is. however you can have decent success if you come up with a chord progression and then write lyrics over it. you can then even take chords out and write riff heavy music, but having a basic chord structure usually helps establish a musical baseline to write a melody
I always have a stock of lyrics before I write something on the guitar. Takes some adjusting to fit them to the song afterwards, but I think that's good because it structures the song. I also have an additional obstacle, in that I'm one of the two guitarists and the sole lyric writer in my band. That means that I have to try and explain my idea for patterns to the vocalist with every song.
As a bonus tip - it seems that Hetfield had a good idea when it came to writing lyrics. When listening to the MOP demos that came out recently you can hear that they had a song written, and he was just humming along to it to write the vocal melody/patterns and then put words to that. Doesn't really work for me, but it could for you.
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