I start with a mood and build a melody around that with an idea for lyrics in mind. Once the mood is established, I start writing lyrics and the song is completed within about five or ten minutes on average. I typically don't change anything when I am done. My mother wrote songs the same way when she was a country songwriter with a Nashville contract. She usually wrote the lyrics in about five minutes.
Other writers might take months to write a complete song.
I am not a prolific writer, which is my biggest problem. I think it is just that I have to be in the mood to write and that means, I have to be playing before a live audience. When I get too busy, I don't write at all, so, I write in spurts.