I dont know how it is for other bands but for mine it kinds starts like this.
turn up at band practice. Open a beer, roll a spliff.
I might have a new bass line, the guitarist has a new riff and we;ll jam for a hour or so and let a few musical ideas flow. Have a flick thru the lyric book see if any ideas fit the music. If so make a few adjustments and rattle it out a couple of times. try a few chorus changes, a middle eight, a bridge (if any are needed). Once you have all the ingredients then you work on the stucture and arrangement which is just trial and error.
I dont believe in any of that theory rubbish. get a few good musicians together and leave them to it for a while and you can 9 times out of 10 get a template for a tune.
or a song can arise just from sitting home strumming and acoustic and tryingdifferent chord sequences, tempo, rhythms and then hum a melody. Writing decent lyrics is the difficult bit i guess.