This post is all about my personal songwriting process. For the sake of this discussion, this is for my "artist" music, not for music for sync. I know everyone has different approaches, so I wanted to share some of mine.
Almost all of the time I start by writing the music for the song, not the lyrics. I like to have an idea of the different sections of the song, and the overall vibe before putting words to it. The vibe of the music helps me to figure out the lyrical content. Not only do I write the guitar parts, but I like to have an idea of what the other instruments are doing, because it needs to be one cohesive unit.
I record song ideas on my phone immediately after creating them. Many times I have remembered most of an idea, but the essence of the song is lost if the rhythm, or something else is different. I learned to record the ideas when they come to me, whether they are 10 seconds, or full songs. This way, when I feel like writing lyrics, I can sift through my database of riffs and find the one that excites me the most or fits the best.
To start the lyrics, I often like to think of a song title, then write towards it. If I have a title, and an idea of what it's about, it almost writes itself. I can say "Ok, the title is Thrill of the Chase, it will be about chasing your dreams and never stopping". Also, I often start with the chorus lyrics, since the chorus is the most memorable part of a song, and helps me narrow down the ideas for the verses.
Similar to my database of riffs, I keep a list of potential song titles in a text document. Whenever I sit down to write, I can access my ideas and use them as springboards. When I write lyrics, my 2 best friends are a Thesaurus website, and Rhymezone. I'm not ashamed to use these aids, and you shouldn't be either! They give me ideas for synonyms and rhymes if I get stuck on a line.
After the lyrics are written, I figure out the best key to sing it in, and also write the bells and whistles to the song. A song is not as recognizable or cool in my opinion without some nice fills, instrument overdubs, transitions, etc. I am a big fan of vocal harmonies, so I write those and sometimes guitar solos prior to hitting the studio. Before recording, I also decide what I'm doing with percussion instruments (tambourine, shaker etc). It's easier for me to envision the song when I have all of the elements decided beforehand.
One of the last things to consider, is experimentation. Sometimes during the recording of a song, someone will suggest an overdub that I hadn't thought of. One example from one of my songs is the vocal harmony in the "Cloud 9" prechorus. Jared, the engineer for the session suggested a change for the harmony which really added a new dimension to the part. In the same song, I jokingly added a line during the breakdown, which I kept because it was too funny to throw away!
Lastly, I think it is important to play a song a lot, and live with it for a while before recording it professionally. When I write a song, it starts out somewhat basic, but it evolves the more I play it. It is better to record the evolved song than a basic idea.
And there you have it! These are my approaches to songwriting. The process is fun, though sometimes frustrating if the inspiration isn't there. You can't wait around for inspiration though, the more you sit down and write, the better you will get. If you are an aspiring songwriter, try doing it often. You have to start somewhere!