I find it a very personal thing and a lot of factors influence it.
for example, I always found the music started off a song, then a concept came, then lyrics followed.
Occasionally a concept came, I'd write music and the lyrics would then be written.
recently, the last two songs I've been working in I've made a real forced effort to write lyrics with no instrument, then work out a melody, then fit music to it. I should point out how much of a forced effort this has been and I did it to try different approaches and see the pros/cons of it, plus I thought doing something different may open my options/styles for me (it has).
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Other factors that influence me are the guitar I'm playing on when I'm writing, same for the amp.
That's part one - how a song grows can be influenced also, eg: write on an acoustic guitar, play it on an electric the song concept is the same but the feel changes, vice versa too, etc.
My only generic tip is "write the truth" it doesn't have to be real as in something that happened to you, but make it real, eg: if you know something, or think something or feel something, write about it honeslty rather than what you think you should say, it comes across in the finished song, and later in the performance.
It's a personal thing what works for people on writing.everyone is different. Experiment.