For me it's all about the inspiration and the lyrics and that's inevitably where I start; unless it's an instrumental of course.

My songs are very personal and they have to tell a story with characters I can visualise or relate to, a bit like writing a book I guess but much shorter. I will agonise over a single word and I hate lame rhyming or fillers, every word has to fight for its place.

Once I have a lyric then I'll pick up my acoustic and start playing with ideas, typically recording to my smartphone. Or I'll fire up the digital piano and get some chord progressions I like that work, I am no great shakes on piano but I can use it enough to compose a basic idea. If I decide on piano I have been experimenting with EzKeys and I really like it but I need to be more experimental in modifying the default midi clips.

I tend to define an accompaniment then work out the tempo and melody from there. Finally I add drums and other instruments.

My big weakness is my production skills and I am probably only functionally competent in around 30% of what Sonar can do so I have a lot of learning still to do but you can only get better with practice, right? 😂

Here's an example of a song I recently wrote.
Noodling away on a guitar while watching TV, I came up with cool lick.  After playing it a few times, I built a basic chord progression around it and sang a verse (whatever words came to mind).  Opened Sonar, and recorded my playing the chord progression, lick, and sang the verse.
Then I threw together a simple drum beat that felt close, and added a simple bass line.  Then I wrote a chorus, 2nd and 3rd verse, and recorded them.  That became the basic "so I don't forget" recording.
Then I got off the PC and went back to the guitar to re-arrange the song, add a middle-8 section, whatever felt right as a performance.  Then I went back into the Sonar project and updated it with whatever else came out.  Export a copy so I can listen to it on my phone as I do other non-musical things.
At this point, it was a fairly run-of-the-mill blues song.
A week or so later, I was listening and imagined another guitar line working as a counter-melody.  Since I felt the song wanted some brass, I programmed a 2-trumpet/1-trombone section.  Then this re-inspired the other instruments and I rewrote the bassline and drums to match how the feel changed.  Somehow this led my blues song into a weird Klezmer field.
I didn't want it to be a Klezmer song, so I replayed the brass on a guitar.  Then it leaned a little more back towards blues-rock.  I exported an updated version of the song.
A week or so later, I was working on something in the song and isolated one of the guitars.  Suddenly i thought "i wonder how this would work as reggae?"  A quick drag/drop of a reggae drum pattern, a "wing-it" bassline, and there was another life of this song.
 
Now I'm leaning back towards the blues-rock version.  But it will likely involve a little reggae/klezmer by the time I finally complete a mix I'm proud of.
 
Most of my songs evolve like this.  There usually ends up being 3 or 4 versions of each.  Occasionally I'll have the songs transform between genres gradually, but sometimes they just live in different worlds.
 
I used to be a fan of Hawksley Workman, and what impressed me most was that you rarely saw him play the same set twice.  if he was in a hard-rock mood, you got a hard-rock show.  If he was feeling ballady, you'd get a softer, more sedate concert.  And his songwriting and arranging were good enough that the songs worked in whatever genre he chose.