This is a very interesting post with a lot of good advice. I believe that the most important thing when it comes to writing songs is the feel and emotion of the song itself. It all depends on a number of factors of course and the process can vary. One thing that I would mention is whether you are writing alone or along-side more musicians. If you are doing all the work by yourself, things get a little bit easier as you are the only one who has a certain direction in mind that the song should take and adjust that direction and moving back between the instruments and lyrics in order to make them go well together is as flexible as you want it to be and there is no idea clashing. However, if you are writing with somebody else, ideas can clash and they might not share their vision of the song with you. The more people that are involved, the more difficult it is for everyone to achieve their vision of the song and usually somebody has to compromise. Personally, I have never been a very good instrumentalist. Although I can play a couple of instruments on a medium level, most of my knowledge of them is there to accompany my vocal needs. Since that is the case, I usually leave all of the instrumental work to be done by my instrumentalists, mostly guitarists and then I chip in with vocal lines and of course the lyrics. While the vocal lines are always my own, I would usually ask the guitarists if they had an idea for the lyrical subject. If they didn’t have the answer then I would just let my creative side run wild. I would mostly listen to the song while humming the vocal line and listening to the feeling and what images the song invokes in my brain. It could be just one simple thing like love or hate, or something more complicated like an entire story of a certain character, fictional or real. Once I know the subject and the line, it is not that hard for me to put it all in a cohesive narrative and combine those fragments into a whole. Afterward, we practice the song with the band and I make small adjustments like word choice and length of the vocal lines to fit the song better and make things sound interesting and not too repetitive.